CBM 1541 drive faults

I have four 1541's now, two of which seem to init OK, and two of which sit there with the drive spindle constantly spinning and the access LEDs lit, the latter behavior which I believe indicates a multitude of possible microcontroller-related faults... I've done the obvious, reseating socketed ICs, checking the +12V and +5 rails, and checking the on-board CPU reset line. Does anyone have any tips for what's best to try next? I read somewhere that ROM faults aren't unheard of, ditto with 6522 VIA failures. Also the 74ls14 at UA1 (particularly if someone's unplugged the drive from the host with power on), but I think that affects CPU reset, which appears to be working in my units. (is the firmware interchangeable between different board revisions? i.e. can I swap in firmware from one of the drives that appear to init OK to rule that out as a fault, or do I have to worry about matching PCB revisions?) Also, has there been a worse external drive in the history of floppy storage? Slow, complicated, expensive and unreliable seems to have most bases covered ;-) cheers Jules