My '87 has been doing this for a while now. Haven't bothered to troubleshoot it yet. It seems to happen when the a/c has completely cooled the car, and it is starting to get too cold inside. The PBU calls for a little bit of heat to be mixed in to prevent the temperature from falling below the setting on the dial, and BAM! full heat.
I've waited a bit (not long, it gets hot fast!) to see if the heat shuts off, and it never does. Switching the PBU off for a few seconds, then back to a/c mode always cures the problem.
The first thing I'd try is unplugging the auxilliary electric coolant pump. If it has shorted out you will get some wacky behavior out of the PBU. Unfortunately I don't know where the pump lives on your car - it's somewhere under the hood, connected to coolant hoses
On my diesel it's near the firewall, on the drivers side of the engine. I think the location is different on gasoline cars, though.