I also have 300D 1980 .

Vacuum is crucial to it working. It will do exactly what is happening to you
if vacuum falls off.

Under the glovebox side there are one or too loose lines that stay open.
One is black, can't remember what the other one is.

When I developed a slow vacuum problem my climate control stopped
working too.

Fixed the vacuum, fixed the problem.

Double check for a snug fit on those lines.

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80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250