PC is slowing during playing games

[thenamesake]

hi
when i start playing games on my pc after 5to10 minutes game slows down for few seconds frame rate drops from 30 to 4 fps. again frame rate is improves but after some seconds game again slows down and this cycle goes on i.e. improving then degrading again improving frame rate.
my specs are
3.0 ghz dualcore
asus p5gc mx mobo
palit geforce super +1 gb 9500 gt graphics card
3 gib ddr2 ram
410 gib sata hdd
dvd r/w
450w psu
15 amps on +12v railing
2 extra cooling fans
this happens especially extra heavy graphics games like assassins creed, crysis,GTA IV (sometimes this happens in playing card games)
i play these games
with 1024x768 with details on high AA is off
how can i solcve this prob
i also tried this games on extremly low resolution with details on lowest
still i pursuing this prob
is this due to heat? because my side panel of cabinate heated so much
or this due to low power supply i have 15 amps on +12 v railing

 

[BlindObject]

I say you should re-install your display drivers and if that doesn't help. If not, then check your temps and boost some fan RPMs, then finally, try upgrade your PSU, your due for a new one anyway, 450w seems kinda low for me.

 

[thenamesake]

i upgraded my video drivers but nothing happened
what is ur meaning of boost of fan speed?
how can i do it?
i am thinking to add one more fan for cooling and replacing psu
is this occurs due to dust?
please give me advice i m very much in trouble.