A/C eratic

Most likely a vacuum leak. Does it turn hot when under acceleration? If so it's a common symptom of a leak. One of the (many) hoses are probably cracked and causing loss of vacuum to the actuator(s) that control the doors of the climate system, or could be a leaky vacuum actuator that controls the air blend doors. I replaced all my vacuum lines and still had the issue so I had to pull the dash and climate box behind where the radio is and replace one of the actuators that was dry rotted and wouldn't hold constant vacuum. Lots of info about this on the site.

Thanks, aZinMA !

you're welcome.
Check here:
https://z31performance.com/forum/z31…=1466732250360

And just because that actuator that controls the blend door that gives heat loses vacuum, it DOES NOT necessarily mean that it is bad. A leak anywhere in any of the hoses, the rubber bladder in the hot water cock, cruise actuator or the actuators on the side of the heater box will cause that actuator to lose suction and since there's the blower blowing right against the diverter the moving air will help open the door to force air thru the heater core.
So unless you like fun projects, I'd first replace all the rubber hoses in the engine bay and the one going thru the firewall up to the T connector under the glove box and test. This is where using something like a Mity-Vac comes in handy.