Even though your system has passed its diagnostics, you could still be
experiencing a RAM or SCSI problem.
RAM problems can show up after warm-up. For example, a marginal SIMM's
speed may be fine when cool but slow down after warming up. All SIMMs do
this to a certain extent, but you may have a SIMM that crosses the
Macintosh SE/30's tolerance threshold when warm. The only fix for this is
to replace the offending SIMM or SIMMs. A similar problem could be
happening within the memory manager's circuitry on the logic board, the
only possible fix being a complete logic board replacement.
More likely, this is a SCSI problem, particularly if you have external
SCSI devices attached. We have seen this problem most often when there are
external SCSI devices attached but not turned on. All SCSI devices must be
powered on before starting the Macintosh and they must be left on until the
Macintosh is turned off. Doing otherwise can introduce garbage onto the
SCSI data line. Improper termination of the SCSI chain will also cause this
problem.
If your SCSI chain is found to be the source of your problem, be aware that
your hard drive's format may be corrupted (the garbage on the SCSI chain
having corrupted the format). You'll need to reformat your hard drive with
either all the external SCSI devices removed or powered on.