Many MBs have a speed sensor on the back of the compressor. You might use the search function here for it's description/purpose. This sensor will deactivate the compressor if it detects a mis-match. Cycling the ignition is required to reactivate the compressor. Your situation sounds a little like the speed sesnor is at work.
Short cyling can be caused by numerous things to include too much refrigerant or too little refrigerant. Did you test the high/low side with gauges? I believe it's been said here that you need approx. 80% of the original R-12 load when using R-134A.
How much oil was used? I wonder if an excess amt. of oil could short cycle the compressor?
I'm sure you'll receive more answers than mine soon.