Geforce FX 5200: sometimes seems to stop working...

I've got a GeForce FX 5200 with 256 MB DDR of RAM and 128-bit bus; it's on a
Gigabyte KT-400 with an Athlon XP 2200+.
I activated the Direct3D performance graph, and when I start playing games
like the last two NFSs, sometimes the value goes over 200-250 (less than 5
fps?), or the graph become unstable marking a zigzag and the game goes very
slow. I tried to change something in the performance tab, but it didn't help
me and also the quality of the texturing is bad... It's not a problem of the
CPU, it works at 40%.
Before, I had a GeForce 4 MX 440 and it seems I had the same problem.
Don't tell me that my graphic card is obsolete... My friends play these
games with a GeForce 4 MX440 without problems!!! I don't know what I can do!
Thanks.