I'm not sure what you mean when you ask "what DSP?". There is only the one DSP dialog as far as I know.
If you mean which DSP category is active when I see the issue, the flashing happens no matter which is selected (output format, volume leveling, equalizer, parametric EQ, effects, analyzer, convolution, etc.). If I scroll my mouse outside the dialog borders (to navigate down a file list for example) the DSP dialog flashes exactly in sync with the scrolling.
I had not updated my graphics driver for a while, so I did that but the problem remains. The issue arose, or at least appeared to, after I installed MC33, as I never saw this with MC 32. That said, maybe a Logitech mouse driver update is responsible, as those happen in the background. I will try to get that rolled back and see if it fixes things. That said, given the prolific use of Logitech mice, I have to think other people would have experienced the same issue if the mouse driver were responsible, but nobody has chimed in yet with "me too".