Deeply sorry to contact as this thread had been dead for a while.
I have tried the things you’ve suggested but its the same, I have recently reformatted, and when I reinstalled Vegas thinking it maybe the way its configured, no the same things happening. What I didnt add to my post above is I have a AMD Athlon 64 x2 Dual Core 4000+ 2.11 GHz and a ATI X700.
My mate have Sony Vegas too and they can play in Best/Auto he has a Intel Centrino Core Duo.
Could it be they way its configured?
Thanks.