Performance issues suddenly

After 10-15 minutes of playing, very often suddenly I experience big lag, FPS drops to 2-3 frames per second. After game is restarted everything reverse to smoothly FPS but again after 15 minutes it becomes laggy. I changed resolutions (from maximum to minimum and vice verse), every graphic setting from Low to High but result is the same. Broadband connection is enough (cca. 2 Mbit). Any ideas or somebody experienced similar problem?

CPU: AMD64 X2 5000+
RAM: 4GB DDR2
GPU: ATI Radeon, HD3870, latest drivers installed

Thanks.

This sounds exactly like the issue I just posted about here: /111/19/356410/1.html#post2670290

If it helps, my computer specs are:

Win XP SP3

NVidia GeForce8600GT (512mb)

Intel Core Duo (dual 2.4GHz)

2GB Ram

I went from a very fast computer,to a even faster computer in hopes of gaining some performance out of SL,but no dice.,my fps are the same as i had before the upgrade,nothing has changed..
I got an i7 920,X58 motherboard, 6 gigs of memory and a new ATI HD 5870,the fastest single video card you can buy right now,and I still suffer FPS drops and very poor performance in SL only,makes you wonder what you have to do to get SL to work properly.