I combined a 2.0.27 kernel with a 2.0.11 scsi folder, and solved my problems for now. It seems that those kernels that work access the wide scsi disk with 10.0 MHz, while those that fail insist on accessing with 20.0 MHz as scsi-2. In addition to a picky aic7xxx driver that declines to fall back on the lower bandwidth, there might indeed be a hardware problem hidden. I will check termination and wiring again.
However, I would still appreciate an explanation on why the aic7xxx was rewritten to insist on scsi-2, and whether this will be fixed again. During the last two months, I would have happily settled for half the bandwidth, saving me literally dozens of hours of experimentation. regards, b.