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.
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