I have the same problem (renders fine but project stutters) — match media settings doesn’t help.
I’m using HD files from a Canon HF S11 — files are AVCHD, 1920x1080x2, 29.970 fps.
Computer is an ASUS G71x with Q9000 processor(quad processor with 2ghz per cpu), 6GB memory, NVida GTX260m video card, & dual internal drives (sofware on one, project on the other).
It is really annoying because I can’t tell what is happening w/ the transitions (sometimes stutters right through them). I’m new to Sony Vegas Pro … so, not sure if I should be looking at something else.