spikes ro in a good pc

Hello guys ,i get spikes in ro 2,and my pc is very ok for requirements like,i got 50 fps and sometimes get 20 fps for 1 second and later back to 50,this really make me annoyed,the game runs perfect(same game recommended me to play it on high) but dont know why i got that

specs:

AMD Phenom(tm) IIX4 955 Proccesor 3.20GHz

Ram:4gb Ddr 3

Bits 32

Video Card:Geforce GT 640

help please:,C