After tempest and I fixed his we powered mine up only to find a blank screen. Today I reseated all the chips (they are all socketed because I replaced all the chips back in the day when I the board was blown and given to me). Joy of joys, it powered right up today and reported THE TOP OF RAM IS DFFF ! Now it's running a test on all 56K RAM.
Mine also shows a few briefly blanked pixels when it writes to the screen. I think it's normal for the video circuit to blank during CPU accesses to screen RAM. To see an extreme case run the Monitor command TE F600 F700. As it tests screen RAM you see many dropouts (you can also see the test patterns used). When that's done run TE FE00 FF00 and watch the user characters show the bit patterns. Now run TE 0000 00FF and see only a few brief blanks as it tests non-screen RAM.
tempest, is that the "ripple" you see? Or do you see a waviness in the position of thr characters? We looked at your power rails on my scope and they looked very flat, so I think your power caps are fine. I suppose bad decoupling caps (the small ones all around the board) could cause glitches in the display circuit. Can you post a video of the problem?