Finally the ferrite beads have arrived, and I've had the time to try them... and they seem to have fixed the issues with both my 1D4 1200 boards.
Interestingly, one of the boards had a resistor at E127R, but the other had what looks like a ferrite there already. Both were exhibiting the same graphical glitches, so I thought it was worth a shot to swap out both... and both boards now seem to be 100% glitch free. So a dodgy ferrite bead maybe? The original solder still looked good and there were no visible cracks or anything, so I think it was factory fitted.
I don't know if it's possible to measure the old ferrite bead I took off in any way useful (it has 0 ohms resistance on my test meter as expected) but I still have it at the moment, so if anyone wants me to check anything about it please let me know. Something about the ferrite bead seems to have been equally guilty of causing the glitches, as the 27 ohm resistor which was on the other board.
I now don't seem to need to turn on any of the CCK pullup or capacitance options in the Indivison tool either.