Troubleshooting a TRS-80 disk drive

I've just had one of the disk drives in my TRS-80 Model III start to go south on me, and before I crack the case and get down to serious diagnostic work, I thought I'd check with the list to see if the symptoms ring a bell with anybody. I believe that the drive in question is a stock factory install; it's the secondary drive in a fairly low serial-numbered Model III (in the 50,000s). The first sign of trouble was that the drive began returning spurious "Disk is Write-Protected" errors, which would go away upon reinserting the disk. Now it's begun episodes of returning TRS-DOS Error 08 ("Drive Not Ready") messages for all operations. The activity light functions normally, and the drive hasn't made any alarming sounds. It does not spin up the disk when it's in this state. After a while, it'll begin working again on its own. I realize there are any number of replacement drive solutions out there, but this is a system where I'd really like to keep the original equipment up and running until it just can't be patched together any longer. (I can't really explain why I'm so fond of this thing; I just am.) Thanks in advance for any suggestions, or just things to look at first. I've got a scope and logic analyzer I can use for diagnostics, but my knowledge of disk systems beyond the tracks-and-sectors level is marginal.