Nvidia 9500 GT Inconsistent frame rates

I recently bought a Nvidia 9500GT (1gb) and installed it on my friends system [ Amd X2 4600+ , Asus M2n - SE, 2gb ram]. I installed the appropriate Forceware drivers [v.178]. Then i installed the recently released "PRINCE OF PERSIA" and it ran very fine even in ULTRA High settings. I got solid Frame rates. Now I installed the same card in my system. [Amd X2 5000+, Asus M2n - SE, 2 gb ram] . When i installed the game, i was shocked to see that the performance is very low. when the game initiates , for 2min it will run very fine in VERY HIGH settings with Anti - Aliasing and V-Sync ON. Then suddenly the frame rate will drop drastically. and again after 2 min it will run fine... then again the frames drop. Its keeping on cycling. Even if i play in "very low" settings also, its the same. I even installed the latest drivers [V.181.20].

This is happening for every game.I installed all the recent games and the same thing is happening. Then i installed a 4 year old game "Age of Empires 3". Even this is not running properly.For a while it will run at nearly 60fps and next it will drop to 15fps, again it will increase to 60fps... What might be the problem? If anything is wrong with the card, it shouldn't have worked well on my friends system!!!!!!!

Both the systems are Win Xp Sp2. I tried even turning off the antivirus. There is no way to think that the card is not capable of playing the latest games bcoz it will play very well even in the highest settings for a short duration in regular intervals. So what might be the problem??????

I dont think its due to any overheating issues bcoz i checked the temparature with "speedfan" and it is around 45 - 60 C.