MacBook Pro Video Issues

Hey guys. so i have MacBook Pro Aluminum 15' NVIDIA GeForce 9600M GT graphics processor with 256MB of GDDR3 memory on 2.66GHz configuration and NVIDIA GeForce 9400M graphics processor with 256MB of DDR3 SDRAM shared with main memory.

and it doesn't matter which one of the cards i select. every time i watch a HD video it starts going on slow motion (it happens in quicktime and in youtube)

what do you think it could be the issue

also

i forgot to add that the whole computer gets super slow ( i have 2gb ddr3)