If you're filming with a rebel , get vegas 10 or the 9.0d update, helps so much. If it's a gopro, use shift+b because vegas and Mp4 don't go together great. I'm pretty sure vegas 10 works a little bit better with the mp4's though.
RIP Shane & CR
The emporer of china gave me an offer that i just couldnt understand
I never find it's my graphics card for that sort of thing, more your CPU+ram.
That's why having a tonne of memory and multiple cores is needed for editing, because trying to run a couple of effects or filters along with merging multiple setup clips is really stressful on a computer, previewing it in a lower quality will do just that and lower quality + strain on your graphics card, but if it's still fluttering around after the fixes said above you just don't have the power to run Vegas smoothly.