He tried 2 different VRAM SIMMs, though, with no improvement.

You say it takes a little while before they appear; that would seem that heat could be a factor. Right when it starts, try pressing you thumb on each chip on the logic board to dissipate the heat and see if you can get it to momentarily go away. You could find a specific chip to suspect this way. What good that would do to find out is another question though.