Fixed! Again! (but for how long? :-)
Well the recent bout of read errors was two-fold; one, I had
forgotten to solder back in one end of a small electrolytic I'd
lifted in testing; it was on the on-board-generated -6V supply for
the head read amp. (doh.) Lots of noise/ripple I assume raised the
error rate.
I also replaced every single blue-death cap with disk ceramics.
And all of them from .01uF to .1uF (they are all bypasses).
Passes diags, formats, five pattern passes each surface, zero
defects, RDOS installed, tapes reloaded.
We'll see how it goes...