A 91 with only 77k sits a bunch. Fluids sitting cause shellac and varnish to build up on parts which causes valves to stick. Don't blame the trans because of fluid and chemical dynamics. True there are problems with sticky TCC's, but most are on high mileage, abused examples, or those that have sat like yours. I had a 91 Grand Prix with 220k, and never had a trans problem because the previous owner and I did the maintenance properly.