Frequent freezing + errors - possibly related to atapi?

Hello, recently my computer has become nigh unusable due to very incessant bouts of freezing during which CPU use spikes to 100% and the cursor becomes nonresponsive. The problem is somewhat intermittent (sometimes, my PC boots fine and everything runs smoothly; most of the time it does not), but the issue seems to have become more frequent. The system is 2.5 years old, and while I wouldn't say it has run perfectly the whole time (), I did not experience these issues until a couple weeks ago.

First, specs:

AMD Athlon 64 3800
ASUS A8N-E
2 GB DDR400 RAM
ATI Radeon X800XL
2 Hitachi SATA HDD, 160GB and 250GB
DVD/CD+/-RW (LG GSA-H62L; SATA connection)
Windows XP SP2

Now, a long story (apologies, but I figure it's best to tell everything I know so far):

The only new piece of hardware is the DVD drive, which I installed 5 days ago, replacing what I thought was a defective drive of the same model. About 1.5 months ago, that DVD drive had stopped reading CDs, though it was fine with DVDs. I swapped in an old Plextor IDE DVD/CD drive temporarily until I could get a replacement (I had actually replaced THAT drive with the LG one because the former had stopped reading DVDs, and I'd been told the DVD reading mechanism was kaput... yet when I reinstalled the old Plextor ~1 month ago, it managed to read both CDs and DVDs very rarely before finally becoming too flaky to bother with). I ended up putting the LG drive back in, and it seemed to work very, very intermittently while I waited for a replacement.
A couple days ago, I got my replacement LG drive (same model as originally), took out the old one, sent it back to LG, and installed the new drive. I immediately checked the new drive with audio CDs, CD-ROMs, and DVDs, and everything seemed great. Then things got really hairy.
On restarting the next day, the system froze with only a blank screen and cursor after logging in to Windows. Multiple restarts resulting in either this same thing happening, or, if I did get into Windows, the system would typically hang if I tried to access My Computer or Device Manager, in which case it would go to the blank screen with only a cursor. If the system was responding at all, the screen and cursor would freeze for a few seconds every minute/half-minute. This corresponded to regular spikes in CPU use to 100% in TaskManager. I did some research online, and found out I could access TaskManager even if the screen was blank, from which most recommended to run the explorer process. But usually explorer.exe was already running (sometimes twice--odd?). If I ran explorer and waited long enough, sometimes I would get back to the desktop; other times, not. Shutting down, when it seemed to work at all, took 10 or so minutes. I was able to successfully get into Windows a few times, so I ran chkdsk, sfc, and McAfee antivirus; sfc replaced uxtheme.dll, but everything else seemed ok. I had also run the Hitachi Drive Fitness Test on my harddrives about a week earlier with no problems. I've also checked out the various processes listed in TaskManager for suspicious entries.

Finally, I made my way to the system and application logs, and I found a lot of entries like this from before and after replacing the DVD drive, corresponding to my problems with freezing:

Event ID 15
Source atapi
The device, \Device\Ide\IdePort5, is not ready for access yet

Event ID 11
cdrom
Driver detected a controller error on \Device\CdRom0

The DVD drive was then connected via SATA port 4 (which is apparently Ide5 on my system?)

More recently, I also received these 2 together:

Event ID 5
atapi
A parity error was detected on \Device\Ide\IdePort4
(The second HDD is connected via SATA #3)

Event ID 11
disk
controller error on \Device\Harddisk1\D

And these a few times:

Event ID 51
disk
error during paging

Event ID 9
atapi
did not respond within timeout period

I tried changing the SATA port of the DVD drive, but the error followed it from IdePort5 to IdePort3, along with the event 11/cdrom error.
Also, I swapped the SATA cable, but the error remained.

Completely removing the DVD drive, the system booted correctly, and did not record any more errors. However, since I had been able to achieve this (very rarely) with the drive in, that doesn't seem conclusive.

I also heard that the atapi.sys file can sometimes act up if one has Daemon Tools (which emulates an optical drive). So I uninstalled this program, reconnected the DVD drive, and booted up. I was able to enter Windows (since running sfc + chkdsk, I haven't had any problem getting INTO Windows, but I am still getting those frequent freezes and 10-minute shut-downs on some logins, along with atapi errors.) Again, I received errors: first, the disk error (11, controller error), then a sequence of alternating cdrom (11, controller) and atapi (15, not ready) errors.
I have now removed the DVD drive again. I successfully logged in to Windows and checking the system error log I see the disk/11/controller error and one atapi/5/parity error, plus several instances of a disk Warning (51/error during a paging operation). Otherwise, the system has yet to lag massively.

Very sorry for the huge post, but I will be more sorry if I can't figure out what's wrong.
I heard that some folks with atapi problems (not exactly the same as mine) had success by deleting the atapi.sys file and letting Windows replace it or replacing the XP SP2 atapi.sys file with one from XP SP1. I would like to maybe try this, but I am afraid Windows won't be able to access my HDDs if I do this?

Please, all advice is welcome!
Thank you very, very much!

 

Well, I've only had the drive for about a week, so I haven't tested much without it (except prior to receiving it, of course )
I did manage to get things running normally with an old IDE CD drive connected. During that time, on another's advice, I reran sfc, this time clearing the cache first and using my XP CD to repopulate. Sfc didn't report any replacements this time, though.
After that, I reinstalled the DVD drive, using a different SATA cable (I had tried 2 before). I've had 2 successful logins now with the DVD drive installed, but I did get 1 error, reported once in the log:

Event 11
disk
The driver has detected a controller error on Device\Harddisk1\D

I guess I should keep using the system as is, and if the problem crops up again look for updated drivers? I'm not sure I want to update the drivers at this point, because in the past I had BSOD problems (related, no less, to an older Plextor DVD drive!) which I solved by replacing the nvidia DMA drivers (my motherboard is nForce4-based) with the default drivers from MS.