Hi! I have an 87 300D with a similar
(but somewhat different) problem.

I just got my car back from a shop in
Santa Cruz getting my A/C leaktested,
fittings tightened, and the system recharged.
Blew nice cold air! ... Until I hit highway
17 on my way to Oakland. I started climbing
the hill and the air suddenly smelled
damp, then became outside temperature (80-ish).

I got pretty pissed, so I pulled over, cooled
the engine a minute, shut off, restarted.
The A/C worked again.

Then the A/C worked fine on a flat surface,
but when I started climbing the hill again,
it cut out. I was glad I had a 12 pack of
Coke next to me on the front seat, as it
was easily 100 degrees in San Jose.

The serpentine belt and tensioner are 8k
miles old.

If I just leave the car sitting stationary,
the compressor works fine.

Do I need to get the clutch adjusted, or
is the relay suspect? (Are either expensive?)

Warmly,
Justin Dobbs