Resolved PC won't power up intermittently

First the scenario -
Gigabyte m/b new 2 months ago.
650w PSU new 1 month ago as the existing 500w PSU failed (could this be a clue - read everything below?)
Increased ram from 2Gb to 4GB 2 weeks ago.

The problem -
Two days ago PC would not power up (no lights no fans).
Disconnected 'power' plug from m/b and 'shorted the 2 pins'. The PC powered up.
Suspected the power switch but later powered down, reconnected power switch to M/b and restarted ok.

Today attempted to power up PC using power switch. Case light came on, CPU and PSU fans started but NO POST 'beep' and NOTHING displayed on screen.
Suspected 12v ATC connection to M/b. Checked voltage at pug - it was ok.
Started to disconnect hardware 1 at a time starting with second hard drive which has an IDE to SATA adaptor (the M/b does not have an IDE port).
Attempted to start PC. POST 'beep' and initial display (bios) stopped after display of RAM before displaying hard drives etc.
Pressed DEL to enter BIOS settings - very long delay.
BIOS would not even display that I had any drives connected (even though primary (sata) HD and DVD were connected.
Selected LOAD FAIL SAFE SETTINGS and shut down.
Reconnected second (IDE) drive (with IDE to SATA adaptor).
Powered up. POST beep, all drives detected and the PC started up correctly.
I alloyed the PC to remain powered up for at least 30 minutes, powered down and then powered up and it worked ok (maybe because it was at operating temperature).

Any ideas what could be happening?
Could the addition of another 2GB ram (same manufacturer and speed) cause these problems or is it more likely to be a bad connection of the 12V ATC connector?
Should sata devices be plugged into the motherboard in any particular order (M/b sockets labeled 0,1,2,3)?
Any ideas/help would be appreciated.

 

Unplug the NEW stuff one at a time (starting with RAM) and see if you have the problem.

 

I will have to wait till the next time it won't power up as since my post it has been shut down and successfully powered up twice (with 8 hours between the first power down and the scond power up so it was cold) and since the last power up it has been running ok for 12 hours.

On thinking of all that has happened in the last few weeks I feel that it is caused by a plug not making proper connections with the pins in the sockets on the M/b as all this started to happen after I replaced the power supply.
Next time it won't power up I will start by disconnecting all drives and extra ram and it that doesn't fix it AND I can't even get past the POST I will try and squeeze the 'female' pins on the 12V ATC plug in case they are not connecting correctly with the 'male' pins on the socket on the M/b as I have read that if the 12V ATX plug is not plugged in then the PC won't even get past the POST.
Am I thinking correctly?
Also can the sata sockets on the M/b cause problems OTHER THAN not recognizing the drives?

 

Take it back to a basic setup, only the things it needs to work. You have two sets of RAM, try each at a time. You can disconnect all drives and see if it will POST. You can leave the hard drives disconnected and boot to a bootable CD.

It seems like it is having trouble identifying some piece of hardware during POST. If it might be a HDD that is the problem, the HDD LED could stay lit when it stops reacting.

It could even be the keyboard, some systems won't start up without a (working) keyboard attached.

Check the basic hardware.

Matt

 

Since my last post (many days ago) the PC has been powering up ok every day and running as long as I want it to.

I am coming to the opinion that the problem was caused by a plug on the power supply not connecting completely with the pins in a socket on the motherboard. And I suspect the 12 volt ATX plug as I have read that if this plug is not connected then a PC will not even get past the POST. The plug does not feel 'tight' and the clip on the side does not engage with the socket on the motherboard and so the plug can be pulled out of the socket without having to press the clip which cannot be right. The (new) power supply is not a generic one and so I am very surprised.
Anyway as the PC has continued to power up successfully for many days now I will close this post.
I am not 'game' to touch any of the cables in the PC case for fear that the problem will reoccur. I will only ever touch them if the problem reoccurs or I have to change anything.

 

Sounds good, especially if it is working.

Yes, I too have noticed that the clip on the power connector might not be able to seat all the way. We just need to ensure that not too much pressure is applied and it cracks the board.

Matt