Pressure is normal.
You have several possible problems.
One is that the speed sensor isn't indicating the compressor is turning, so the climate control turns it off. This is usually the result of a bad belt or belt tensioner. If the belt is missing most of the rubber ribs, it wont' pull the compressor, so when you first start the car, it runs until the Klima decides it isn't turning fast enough. The relay will shut the compressor off, and it won't start it again until you turn the key off and restart.
The other possibilities are worse --
Speed sensor gone (rare unless this is a recently rebuild compressor and the rebuilder didn't do it right)
Wrong compressor for the car if replaced, so the Klima doesn't get the correct signal.
Wires bad, speed signal won't get to Klima.
Climate control shot, Klima shuts compressor off.
I'd bet on the belt or tensioner, myself. I think my 300D is doing the same thing, just need to get out and change the belt.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!