Could be flex disc, but when R is going, you can get that same effect.

Usually when R is going, it is usually preceeded with a time delay going into R for a while.

Check the fluid first.
And search No reverse...tons of info b/c it is the first complaint on these trans w/miles on them.

I did a few with a hopeful filter /fluid change when they started showing signs , but it did not help.

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A Dalton