i had a 89, very similar. the temp i would say might be fine, verify proper temp with an ir gun on the thermo housing when running at normal temp. also every time you mess with the system and let air in it has to work back out. which is mostly the function of the rad cap so i would replace that too they are cheap. mostly i just think the gauges are terible, i replaced mine with aftermarket the stock one wasnt even close.
for the ac if it is holding pressure its likely electrical(switch or wiring) a good shop should have been able to put a meter on it and see whats going on, maybe take it somewhere else.
1998 Bonneville SSE
2007 Monte Carlo LT-KYB, Clarion, Polk audio, Kicker
1999 Yukon SLE-KYB, Air lift, Reese dual-cam, Tekonsha
1987 Firebird-355/th400-nothing stock