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.