The trans in my former TE has started flaring intermittently going into 4th. I've been advised by an expert that it is likely the failure of a couple of seals on a rear support for the mainshaft that seal against the inside of the K2 clutch drum. These seals are accessible after you have removed everything from the inside of the transmission case.
Hopefully this isn't your problem. I wish it wasn't mine.

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Fourings

84 Audi Coupe GT with Turbo engine, EFI (buh-bye CIS), Jamex seats, Koni's/H&R's, e-code lights. 210k and counting.
1992 Anthracite 400E, 161k, "Grey Ghost".
1986 VW Quantum Syncro, 192k, gone to the crusher in the sky.
1989 300TE 172k. (Was mine, now also Mom's car)
72 280SE 4.5 (Mom's car)