4830 lag in counter strike: source-only on certain maps

system: e6600 core 2 duo

2 gb ram

radeon 4830 @ 1680 X 1050

250 gb hd

fresh install of windows 7, updated drivers



i will be playing de_nuke or de_inferno ONLY and get very noticable, annoying lag the whole time(my fps drops to 30-40 and my ping sky rockets to 200-400). any other map is perfectly fine. i just wiped and cleaned out my old drivers and installed the new catalyst update from today and no dice still. any ideas?