Bad video card?

I'm trying to trouble shoot my system.
In motion my project does not display properly
(its either completely black or sometimes colorized and pixelated)

BUT when i export it comes out fine, which leads me to think its my video card?

my system- mac pro 2.66 intel dual core, running motion 4 on a ATI 3870.
did a clean install of motion without any luck.
trashed prefs and all that troubleshooting 101 stuff.

debating on buying a new video card since i cant figure out what else it can be
at this point! any insight/ideas would be great!
thanks
shawn