Mitsubishi hyper heating unit not really heating

I had my hyper heating units installed about 6 months ago. One seems to have developed a problem. MUZ-FX18NLHZ-U1 is the outdoor unit, MSZ-FX18NL-U1 is the indoor unit.

It used to blow nice hot air all day but the past few days, after I turn it on, it’s been blowing hot air for only about an hour then the rest of the day it blows out slightly warm, almost cool air. It isn’t reaching temp and maintaining it because the room is sitting at about 60 degrees when I have the unit set for 71, so it’s not even close. When it is blowing the coolish air, it will only do that for about 30 seconds or so then stops blowing out anything. Then maybe 5 minutes or so passes and it’s back to blowing coolish air for another 30 seconds.

If I turn it off and wait about 20-30 minutes then turn it back on it will blow nice hot air again for about an hour then back to the same behavior. It is definitely cold outside, this morning it was 1 degree but these units are rated for reliable heat down to -13. Meanwhile my other 2 units keep on putting out nice hot air all day so I don’t think outside temp is the cause.

The troublesome unit also doesn’t seem to make noise like the other two. The other two will periodically stop and I’ll hear what sounds like boiling / hissing and rumbling, creaking and gurgling noises, then they’re back to outputting hot air. I checked outside and nothing seems to be wrong with the unit, the fan spins, the drain hose isn’t clogged. it isn’t iced over or constricted in any way.

Any ideas what a homeowner can try before I have to do a service call?