Just pulled the Matrox 450 dual head out of my edit system and put the ATI AIW128 back in. Tried rendering the offending timeline from last week, and the first render to an AVI file had no image flicker / shudder / jitter / horizontal banding that i was asking about for the past week.

Anyone aware of Vegas having problems on an Athlon / VIA chipset / Matrox 450 dual head / Win 2000 system?