Intermittent Heater Problem

Hello,

Have a 1990 Olds Cutlas Cierra V6. During the coldest winter in the past 30 years my heater decided to play games with me. It takes forever to come on if it does at all. Then it when it comes on it can get really hot. However, after blowing hot air for only about 5-10 minutes it will then blow cold (not lukewarm but as cold as the outside air). Doesn't matter if I slide the lever to 'heat' or 'vent'. I had the heater core blown out but that only gave me better heat when the heat decided to come on. I didn't replace the therm because it didn't seem like therm symptoms.

Is there some kind of diverting mechanism that could be broke that is simply not letting the hot air into the car? Any suggestions are appreciated.

Thanks.