I assume that you turn the temp wheel all the way up, and get good heat?
Then, when you return it to approx 75 degrees, it randomly blows cool air?
I assume you mean 300SEl?

try a couple of things:

1. Check your monovalve in the heater line. Roll down the windows to cool the interior, and set the temp on auto heat at approx 80 degrees. Put a finger on the top of the monovalve (near the battery), and you should feel it click strongly about every 5 seconds. You can also actually grab the heater hose on the passenger side of the engine, and feel the monovalve surge every 5 seconds if it's working properly. these do randomly stick when old, and that shuts off the hot water. When you turn the temp wheel all the way Hot, the monovalve is switched full open and does not pulse. So, that may be your problem.

2. check to see if your in-cabin temp sensor fan is working. find the in-cabin sensor grill, next to the sunroof switch in the overhead console. There is a small fan that sucks air thru this grill to feed fresh cabin air to the in-cabin temp sensor. if the fan fails, the sensor can't detect proper cabin temp, and directs the system to do weird things. Take a small square piece of light paper towel or tissue paper, and press it up to the grill. If the paper is sucked up and stays, the suction fan is working. Cigar smoke or similar will function for this test also. The fan is way down below the glovebox above the right kick-panel. You gotta remove the kick-panel, knee pad, and glove box liner to get to it.

Good luck,
DG