1986 420 SEL ACC Haunted

[Randy]

OK, I'm wafting along on a 15-mile trip this evening; the ACC is blowing cool, but sufficient (has been converted to R-134-A; mistake), with the control wheel set just shy of MIN and the fan control in AUTO. Nothing happens on the first half of the trip; on the 15-mile return, about half-way home, the ACC decides to go to HEAT and revs up the blower to MAX. About the time the blower gets to MAX, the heat starts to fade, then fades completely in about 1 minute, then the blower gradually (about 1-2 minutes) settles down, then it's cooling as before. It did this again about 5 minutes later, same routine. Please comment/advise.