The lag in Half-Life 2 (Half-Life 2)

I have Half 2 from time to time (about every 5 minutes) strong brake as much direct slide show.Play at maximum speed in resolving 1200Х1020...this was in C&C Generals.
Amd Athlon 2 X4 2.9 ghc
4gb ram (3.25 gb)
geforce gts250 1 gb
win7 X32