Thanks EvilCapitalist and Crutch! As Gorgonops said, I goofed in my original posting. The SCSCI2SD board was positioned so as to allow SD card access through the "expansion port" on the back of the case. As you might imagine, this was placing a fair amount of pressure on the 5-connector harness, which is attached to the XCEED yoke board (among other things). I removed the SCSI2SD when I connected the old SCSI disk, so I was performing a two-variable experiment, and likely drew the wrong conclusion: This morning I tried running a clean 7.5.5 install with no XCEED driver from the SCSI2SD, and it ran just fine. This is consistent with the behavior that everyone else observed. Of course the PRAM contents have also changed, so I still don't know with absolute certainty what was causing the problem, but I
strongly suspect that it was the poor positioning of SCSI2SD board. I will be careful not to abuse that harness in the future
While I have your attention, my son and I are now playing with the XCEED drivers trying to get some control over the gamma settings, and it's a bit flaky: With both the 1.0 and 2.01 drivers, selecting a setting other than the one that's currently selected (initially "Custom Gamma Settings" or some such) causes the previously selected setting to vanish from the list! The only way I've found to get it back is to swap drivers (in the control panels folder) and reboot. Also, with the 1.0 driver, pressing option-"Options" does not bring up the gamma setting window. With the 2.01 driver, the settings panel does come up, initially populated with the current parameters, but they quickly zero themselves out of their own accord, and the resulting panel does not let us make new gamma configurations. Simply put, the gamma configuration panel doesn't worth with either version of the driver versions that we tested. We have not yet tried versions 3.0 and 3.5 (which were distributed with the fancier MacroColor 30 HR cards), and have thus far failed to find any information on the topic on the web. Do you folks have any sage advice? Thanks.
Oh, and to answer EvilCapitalist's question, the SE/30 has 68 meg of memory (four 16 meg SIMMs and four 1 meg SIMMs), and a SCSI2SD board in lieu of a SCSI disk. Also we replaced the original fan with a Noctua. Otherwise it is bone stock. It's running the original ROM. The logic board was expertly recapped by Mac84 (Steve).