c3 transmission question

I have a 76 smogger that I have had since new. It has about 120K Miles
on the clock.

I have had the xmission oil changed twice, last one about8-10 years
ago and maybe 15K miles ago. At that time I had it flushed at the
dealer - a fairly expensive proposition if I recall correctly.

My problem - When the beast is cold, it will not go anywhere for a
little while until it warms up - the transmission does not seem to
engage. Once it is warmed up it works fine - no slippage, fairly solid
shifts, etc.

Now I must admit it does not get a lot of use now, so maybe that is
part of the issue.

I am suspecting maybe something needs to be changed or checked, but I
doubt it is a new tranny. I am thinking it is a case of something
sticking or being a bit clogged. Or maybe something run through a part
to be loosened.

Any ideas? I have to do a fair amount of work to it this year (new
composite spring, shocks, brake seals, etc), so am just getting my
ducks in order for the various things I need to check and repair.

TIA - BSA.