Weird problem with trees

[gamera3 0]

hi guys, i have a problem here ( )

My game is working ok in my dell inspiron 9400 (dual core, 1 gig ram and geforce 7800) normal settings

No problem in framerates even in big battles

But the thing is that everytime i try to zoom to the trees with a scoped gun the framerate goes crazy, then i put the normal vision and everything comes back to normal (patched 109 or not, it's the saem problem)

i know that my specs arent good, but the trees are supposed to give me so many problems? even with lowplants?

i have another computer (single core, geforce 8600 gt) that lags a lot in the beaten dog mission but don't show any problem with the tree zooming

any help?

Thanks in advance