OK, I ran PIX and here are some results. First, I selected "Statistics for each frame, using counterset" with a custom counter set which displayed FPS and %Processor Time. I got this:
Yellow is %Processor Time, red is FPS. So if I understand correctly, virtually all of the frame time is consumed by the CPU. On the other hand, FPS oscillates wildly between 60 and below 25. That seems consistent with the flicker I experience (it doesn't happen all the time; it fluctuates).
Then I closed the test program and looked at the timeline:
So it would seem I'm CPU bound, which seems to be consistent with %Processor Time being at 100%.
First of all, is my analysis correct? Second, why does FPS vary so wildly? And then, how should I go about looking further into this? I thought of enabling draw timing and looking at a frame where instantaneous FPS is low, to see which calls consume more time.