Heater warm then hot and cool all by itself

96 Subaru Outback I LOVE THIS CAR His name is Rom! When the heater comes on it gets warm…then pretty warm…then hot…then cools down all by itself. The thermostat stays in the middle (you know the idiot light). My mechanic checked it out but it was working alright when he tried to find the problem. WHAT SHALL I DO?

Does it cycle? hold cold hot cold… ?

If so how long between cycles?

Is the heater fan running all this time?

It takes awhile to warm up (always has) then sometimes it stay warm with no change but sometimes after it’s warm, it gets warmer and warmer until it’s pretty warm, almost hot…sometimes I turn the fan down on low or off it doesn’t seem to matter if fam is on or off it will start blowing or feeling cooler until no heat comes out at all. There doesn’t seem to be any certain thing that I do or don’t do…it has a mind of it’s own.