I want to thank everybody who responded to my post about
chattering against the heads for certain oxides on RTR
tapes. I read about the "bake" solution, but I am pretty
sure my problem was not that because I experience no
loss of oxide on the heads or guides. Nevertheless
I did try "baking" the tapes but the problem persisted.

What worked for me, and I would regard this as a last
ditch effort, was to soak a cotton ball in silicone
lubricant and wedge the ball in the tape path behind
the first rolling guide and the tape heads. The cotton
ball provided a small amount of lubricant, but it also
provided increased drag from the supply side which,
I think, increased the contact between the heads and
the tape. I was able to read about a dozen rare tapes
this way. Maybe the solution will work from someone else.