There are a couple of things to look at.

First, a faulty engine temperature sensor can cut the A/C out on that car, but it doesn't seem likely in your case. (It is a separate sensor from the one that supplies the ECU and has only a single wire running from it. Just check the resistance cold and at several points in the warmup of your car. It should remain constant but I can't remember at what value)
Second, the refrigerant pressure switch can cut it out. If you have an overcharge, acceleration could build the pressure to the point where the switch cuts the A/C out until pressure drops to an acceptable level.
Third, the compressor speed sensor on the back of the compressor can sometimes be problematic.

I wouldn't be too quick to blame the relay. I owned an '85 190e and had issues with the A/C, but they turned out to be relatively simple once I understood all the things that tell the A/C to work or not.

__________________
'96 C280 (gone)
'97 C36
'05 C230k