Heater problem
1992 Buick Roadmaster V8 Two Wheel Drive Automatic 86,000 miles

I,ve read previous threads about heat issues with roadmasters on auto forums but I'm still getting no heat. Our 1992 roadmaster 5.7 has 86,000 miles on it. A/c worked all summer but now with heat needed where getting nothing. I replaced the thermostat twice (failsafe first time,then 195 degree the second) Temp gage still rises above 220--will not steady at 195 ?? Flushed out system three times--clear water with no sludge. Took off the heater core cover --core is hot with no leaks.Core is getting hot water and all the vent doors work properly,also replaced heater control valve ,still no heat. Elecronic heat/ac controls seem to be stuck on a/c-- I select 90 degrees on heat/ac controls with the economy button select as well and recieve outside air with no heat,also when I press "auto" temp selection the a/c kicks on and blows a/c cold air. What I'm trying to explain is that I believe the a/c /heat controls are not swithing back and forth from cold to hot like its stuck on air condition mode.Whatever temperature I set the controls at ,I still get A/c cold air even at 90 degrees the a/c compressor runs continually ( outside air is 65 degrees).Any suggestions??? I,ve tried all the ideas presented in this forum,I believe? Maybe I,ve missed somthing. Water pump?? heater/ac relay,temp sensor??
thanks