Need Help with choppy framerates

Just put together a new pc.
When i run any 3D app in FULLSCREEN the graphics stutter, like every few seconds, even when the cpu/gpu isn't being pushed <60%.
This however doesn't occur when running in windowed modes.
any suggestions appreciated 'cause this is really annoying.

spec:
core 2 quad q6600 (oc'd 3ghz 333x9) 1.2625v
powercolor pcs radeon hd4850 (665gpu, 993mem) 512meg
4 gig Corsair 800mhz (5.5.5.18)
Asus P5QL-e
Seasonic S12II psu 500 watt.
Vista home premium 32bit.