When I render an .avi with Lagarith (save alpha, remove black) with a stage size of 876x480 (and a removed background image/.avi) everything renders fine - PI FX are smooth and all lines up on my Vegas timeline with the background.
When I render an .avi with Lagarith (save alpha, remove black) with a stage size of 1280x720 (and a removed background image/.avi) I get severe flickering of the PI FX on the Vegas timeline.
I have tried HUFFYUV, I have made sure to REMOVE the background instead of just turning it off, I have the most current drivers for my video card (nVidia GeForce7900GS), I have tried saving the PI FX file and rendering it in PI Render outside of PI. I have rendered inside of PI with the stage zoomed to 100%.
In theory there's no difference between theory and practice but in practice there is.