Modem Question.

I have been with bt broadband for a couple of years now,as part of their setup pack I got the standard ADSL USB modem.This worked really well apart from one annoying habit,if I was ever using a P2P program (eg. Limewire) my connection used to freeze on and I would have to press the reset button and then close down,if I didn't press the reset button my machine used to freeze at the 'windows is saving your settings' screen.I decided to buy an internal ADSL modem.After setting it up it was working great,no freeze on when using P2P applications,I was happy.I then uninstalled the old Bt ADSL USB modem drivers,later on I discovered that my download speed was half what it should be,I thought "what has changed? ",the only answer was that I had uninstalled the Bt Modem drivers.So I installed the BT modem again,then set up the internal modem as my default connection again and it was working normally once more.(I tried this twice more just to see and both times my download speed halfed).How come my internal modem will only work to the correct speed while the OLD Bt modem drivers are installed on my PC?It doesn't seem logical to me that drivers that I'm not using affect the performance of my internal modem.
When I connect to Bt does it somehow detect it's modem drivers aren't there and that affects the speed?

I am just curious as everything works fine as long as I leave the old USB modem drivers intalled.

 

OK,I thought it sounded illogical in my last post,I've noticed that after using a Bittorrent program my download speed is down,after a few hours it gets back up to the normal speed when I download things from the internet.Is there something I can use to flush out the modem after it slows down?
This modem never reaches the proper speed on some of the broadband speed test servers even when it's downloading at the proper speed.The speed test tells me my download speed is 95 KB/s but i can download something right after doing the test and it downloads at a steady 180KB/s.

Can cheaper modems be off with the speeds sometimes?

Do you think I should buy a more expensive internal modem??