I had exactly the same problem with a 1981 Horizon.  Third gear would
just disengage.  Engine would rev up.  Kind of disconcerting.

I sold that car quite a few years back but the memory of that tranny
sticks with me.  It also had a clutch chatter in first that the dealer
could not fix.  If the lemon law had been in place then, that car
would have been covered.

I have had several Jap cars since then (figuring the Horizon was my
contribution to the American Auto Companies), and have never seen any
bad behavior with the exception of a Toyota Tercel with a bit of
clutch chatter that they did fix on the first try.

Anyway, from that day forward, I have sworn that I would never
purchase another American car with a standard.  American manufacturers
don't have a clue on how to manufacture five speed transmissions and
have been doing the automatics much longer and on many more cars.
However, I hate automatics, so I am still buying Jap cars.

Not sure this is any help, but other cars do this too.

Cheers, Larry
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    @@               Larry Rogers               *
   @@@               larry_rogers@dg.com        *   Big Brother
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    @@ &&            Data General  508-870-8441 * 

The opinions contained herein are my own, and do not reflect the
opinions of Data General or anyone else, but they should.