Hey guys, just for "closure" (as often there is none in these threads), I chose to sell the 2nd gen C4 I had bought as a stop-gap (per my original description). Son limped the car up from LA and I set about re-building his original C4 first gen tranny over a 3 week period. Never had done that before, so I was quite uneasy. Took my time, took a zillion photos, and found at least 3 significant issues that would have explained the bad behavior.
Anyway, C4 is back in, new servo, o-rings, bands, clutches, steel, thrust washers, bushings, seals, etc. and the Stang is back on the road. Only set us back the price of the master rebuild kit so I'm going to call it good for now. I'm sort of trivializing a rebuild that my hair standing on end at times, but it all worke dout.
Thanks again for all your input.
Tom