It sounds like you have a CCU problem and possibly an aux water pump problem. 3 years ago my 86 300sdl had the same issue that you have. I would get heat and then it would just disappear. I could turn the car off and then back on and I would have heat, for about 2 minutes, then cold again. Here is what I found. My aux water pump was drawing too much amperage due to its old age. It would then make my CCU overheat and on the 86 and up 126 CCU's, they shut down and go into defrost mode. These units have a circuit breake built into them and turning off the car and restarting it would reset the circuit breaker till the unit got too hot again.
I fixed this issue by first checking my monovalve, it was good. I replaced the aux water pump, but the damage was already done to the CCU. I replaced the CCU with a used one from a wrecking yard. It did not work. I ended up going through 3 used CCU's and they all had issues. I finally bit the bullet and bought a remanufactured CCU and all was well and continues to be to this day.

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John
1986 300sdl
1985 380se