I'm guessing that you have an M40 tranny and carbs. If this is a tranny
or driveline problem (which I doubt) it would probably be a bearing in the
process of disintegrating, locking up and breaking free. This could be
in the center bearing as well as in the tranny. I would have an
easier time thinking it is an intermittent ignition or maybe carburetion
problem. My 164 did that once and it turned out that it had backfired
and shattered a very old, brittle air filter and that pieces of the
filter were holding one of the carb pistons up. I resolved to pay a little
closer attention.

I'd say see if you can get it to do the same in second and fourth gear.
If so, you have pretty much exonerated the tranny. A worn distributor,
a vacuum leak in either the carb or the vacuum advance, poor point settings
or even water in one of the carbs can also produce the symptoms you describe.
--
George Downs Bartlesville, Oklahoma