I had an old thread on pac-man not being displayed properly and since I've done a lot of checking I decided to start this new thread as the symptom has changed.
The screen does respond (with different garbage) when i press the 1P button to start the abc diagnostic program so I'm confident that the processor is executing code. I wish I could post a video showing motion but. The area highlighted in yellow drop straight down from the top of the screen in columns represented by the junk spaced out in the yellow highlights. The area below in blue moves horizontally. Has anyone seen this sort of error.

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What I've done so far is to replace or swap chips in the video stream starting at 3E & 3H and 1E & 2E across to the output at 7F. I've used my multimeter to check for continuity on all traces in those circuits or the buses they touch which was a week worth of fun. I also checked them against short to ground or +5. I verified that all pullup resistors were in place and of the correct value. I went back through the clock circuits on the left of the schematic to verify good clocking for the primary V & H clocks as well as the derived signals such as vblank, hblank, etc. All were good. I did check to make sure the processor was not being reset. I've looked the board over for solder bridges and none were obvious. I've looked at mikes arcade and lawnmower man web sites and didn't find a match to my problem.
I don't have a fancy logic analyzer, just a cheap 8 channel USB connected one. I have a digital oscilloscope, logic probe, and multimeter. Beside checking for signals that should have data that are either high or low what else should I look for, especially on the various buses. I understand how to verify operation of the single logic ICs like and, nor, nand, etc but not how to check the data buses are operating properly with the equipment I have on hand.
I've been working with digital circuits as a hobby since the Z80 first came out but this is only my second and last arcade repair that I'll attempt so advice from you arcade experts will be greatly appreciated.. How would you approach dividing and conquering the circuits to find the problem?