I have switched to Vegas 64-bit with W7 and i7-2600 CPU @ 3.49GHZ with 8GB RAM and 3 wide-view Samsung monitors.
My project uses only uncompressed AVI clips rendered to Vegas from TVPaint at 1280x720x24 fps. The only FX I use are automatic dissolves, timecode display, plenty of Sony Text clips -- nothing else, and around 10 to 15 sound layers. My project is about 60 minutes long at this point.

Here's my question: My playback picture quality is excellent when preview quality is set to Best (Full) -- but Vegas doesn;'t seem to have the option of locking that setting. During playback the quality will drop to half and back to full; at random. If I wanted that, I would set my preview window to Best Auto. I want Best Full to hold no matter what the playback frame rate does. Is there any difference between the Auto and Full settings?

I also encounter flashes of black or white frames during playback; most often during dissolves or when the picture contains a wide, flat, single color area, such as the sky or a field.

I never had any of these incidences happen to me in Vegas 10-32 bit -- with a much lesser computer; in other words, I am not thrilled with the performance of all my upgrades. Can someone suggest where/which setting I should look into to correct this situation?

This is a "feature" that was introduced in Pro 9.
Right-click the Video Preview window and select Adjust Size and Quality for Optimal Playback if you want to emphasize frame rate during preview.
When the command is not selected, video quality is emphasized, and the frame rate will be reduced if necessary.

Mike, I honestly don't see that option under any of the tabs -- is it possible that it has been left out of Vegas 64?

Paul, it's not under a tab.
Right-click in the Preview window and you'll see it.

Thanks, Mike! This did it, but what's amazing is that I had it checked and unchecking it fixed my problem because not it runs at Best (Full) and at constant 24 fps. What I had neglected to mention is that this happened to me only when I was previewing on a secondary monitor; not when the preview window was running alone.