1990 900S - Heater works only intermittently

[g10sha]

As the subject says, the heat works only intermittently. When it blows hot, it is really toasty. However, without touching the thermostat, the air will suddenly go cold.

Any ideas, or known issues?

[Oil_Eater]

There is a brass coolant vent where your upper rad hose meets the block. with the engine hot and running, slowly open the vent to allow air to escape. Be sure to watch your coolant expansion tank level while you are venting air.

If this doesn't help the problem, you may need a radiator flush to properly open the passages in your system again.

Good luck,
Jay