About 90% sure U26 may be faulty. Before replacing it altogether remove the chip, clean the pads with braid, followed by some cleaning with alcohol, then reattach the chip and we’ll see if that solves it. I say this because that’s the video RAMDAC chip. It’s possible there may be cap goo from C29 under the chip.Whelp, the fresh VRAM showed me that it probably isn't (just?) the vram to blame. The symptoms have changed a bit - the screen will now eventually start to flicker with different colours. If I move the mouse cursor to select a menu (blind) you can see obviously correlated changes to the flickering and bands of colour/black on the screen.
I'm pretty sure that I have verified the continuity on the VRAM SIMMs to what look like sensible chips on the board (CLUT & MEMCjr).
The board boots to this state with and without a RAM SIMM in place. I can hit keys and hear audible responses from the OS error chime and even hit the power button followed by return to power the machine off.
Does anyone have suggestions as to what to try next? Another reflow of something and/or removal / cleaning of the VRAM / RAM SIMM sockets?