Gentlemen,
Thanks much for the above info, it was all very helpful.
So, I now have some hard info on a real error. I read all my uCode ROMs, and checksums match on all but one. U340 has errors as compared to 160-0338-00. Locations $26D, $26F, $2EC, $2EE, $322 AND $3A4 have 1 bits in places...