You may possibly be overcharged with r-12. Gets hot pressure goes up, trips out the compressor, you fiddle with the buttons giving it time to cool down. and repeat the cycle. I would have a high index of suspicion about this since you recently had major work done on the system. Have the pressures checked with a reasonable heat load and I'll bet the low side will be above spec.

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Jeff Lawrence
1987 300e
1989 300e
1987 BMW 325