Your comments about the 8302 make it highly suspect.
The 8302 is on the data bus, plus A0, A1 and A5. It receives keyboard input from the 8049. If active when video is being read out, it would create this type of shimmery stipple pattern, and its data connections are tri-state. I've seen a lot of these with heavily tarnished pins. At Sandy, and even today, I use a gritty pen eraser to gently clean each side of the pins, while they are supported from the other side by the table or a blade. Some would just.... Suicide.
Places I routinely check for weirdness are:
Pin 8 /DSMC
Pin 10 /PCEN
Pin 26 /IPL1
Pin 27 /RESINT
Pin 28 /RESET
Any traffic on [D0:7] by 8302 when 8301's Pin 38 (VDA) is supposed to be asserted. That is, the VDA signal that isolates that bus may be defective, or either of IC19 or 20 (74LS257) may not be properly isolating the bus. When the screen is being drawn, the 8301 and DRAM cut themselves off from the ROM and CPU, but the 8302 is in the 8301's little private party and can scribble over things while this is happening. Which sounds most likely to you?
Meanwhile, might as well get a spare 8302. https://www.sellmyretro.com/offer/detai ... -chip-1822