GPU Problem: While playing games, FPS gradually slows down.

Hi,
My system specs are:
Pentium D 2.8Ghz
RAM 2GB DDR2 533Mhz
Mobo - Mercury PVM7 Pro Version 1.0
nVidia GeForce 128MB 5200FX AGP 8x - No FAN provided by the company
80GB Seagate SATA

While playing games, after about half to one hour, game starts acting too slow. While playing, frame rates are reduced and game runs with pauses. I am taking here the example of NFS Carbon. This happens with all other games too. This happens always, no matter CPU/GPU is overclocked or are in their factory settings. I highly suspect this is overheating problem.
But when I was trying OCing, I installed a 12mm fan on my GPU's HS(it doesnt have any fans). Then I ran stability test for 4-5 hours, then played games for another 2-3 hours(with this problem) and then shutted down my comp directly and opened the case and touched the HS on GPU(all took 1 min), it was dead cold. I have tested this before installing the FAN too, but that time HS was very hot. So, I am confused. Anybody please help me out.

Answer this question too:
How to measure the frame rate I am getting while playing any game, say NFS Carbon? Can it be done through any other way than benchmarking tools for NFS Carbon?

what other programs are running. Is your anti-virus kicking in over the time frame? read the guides forum- there are at least 2 articles relating to your situation.

I think its related to the size of game your using. see my post on the other thread. I dont believe your card has a capability to keep up with the games.

Since the HSF was hot before you installed the fan and cool later, it'd definitely be an overheating issue. BTW, if your card is passively cooled, how'd you manage to get a fan in there? Or did you rig one up to point at the card? Or did it come without a heatsink at all?

This may be an issue. I am using Avast Home Edition, Latest version. All settings are at their default value.

I anyhow made a fan blow air ON the HS, the fan is just above it(<1cm).

And if it is overheating problem then why after installing the FAN too, the problem is not soleved??

I am unable to find any related article in guides forum. Can you please give me the link here????