Yeah Jack, it is strange sounding alright. A check of the water valve insert (rubber bellows) may be a good starting point. The airflow doesn't change, correct? Just the temp of the air changes?
What would happen if you had hot air, then it switches to cold air, then you hit the defrost button without revving up the engine? You could also try this:
Locate the watervalve. If it is heating normally, then switches to cold air, park the car but leave it running (in the failed state), then go under the hood and pull the electrical connector off the watervalve. The default setting (no control to the valve) is full heat, so it should go to full hot air. If it does not do this, I would have to say either a bad valve (more likely) or a strange heater core or heater hose problem, maybe a collapsed hose, but nevr seen it happen before.
But I'd pull the insert out of the water valve first and look for a tear or small hole in the bellows. The insert part has a repair kit available fairly cheap if that's the problem.
If none of this pans out I'd say either a temp sensor problem or a bad push-button panel. (Should be tested)