I suppose it could be a faulty climate contril unit, but for alot less $$, I'd look at the monovalve insert. I'm assuming your yr. model has such a device(monovalve). Check out the search facility here for "monovalve". This device or should I say, a faulty monovalve insert diagraphm is notorious for flaky heater operation.
1991 300-SEL - Model 126
M103 - SOHC
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