Ok, so I first disabled MAPROM on the A3640 - made no difference. (It turns out that I needed to close pins 2 and 3, as there isn't a 4th pin on J400.)
Removing the two 8 pin chips near the ZIII bus allows the computer to boot with the PIV installed, and display video through the 23 pin RGB port, but not through the PIV (I didn't suspect it would).
My 3000 also hangs when the PIV is installed.
Does this sound like something that can be reasonably fixed, or should I begin the sure to be unpleasant process of getting my money back? Thanks again...