Sixto - I fought this problem on my 124 a few years ago - the compressor would drop out. Restarting the car would get it going again, sometimes for only a few seconds.
It turned out to be a multiple point problem. In my case it turned out to be a defective Klima relay, excessive compressor clutch gap, and grease/oil/gunk on the clutch facing. All these together conspired to trigger the compressor speed sensor cutout.
You're going down the right path in analyzing and understanding the input signals to the Klima relay. A few thoughts on this - the signal from the PBU is ground, not +12v. If your tach works consistently, then it's very unlikely you have an issue with the engine speed signal. You can test the compressor by jumpering 15 to 87. I built a Klima replacement from a standard "ice-cube" relay. It used the ground signal from KL to switch on (power was provided by 15). When switch on, 15 provided power to 87. If the a/c works consistently with this replacement setup, you can eliminate the PBU and compressor clutch coil as possible problem sources. I don't recommend leaving this alternative in place - you lose the protection of the serpentine belt by the speed sensor mechanism. It can be a useful diagnostic tool for the do-it-yerselfer, though.