What year is thes? Does't matter too much now as it's at a shop. This could easily be some common ailments like clutch gap, faulty pressure switches. It would be designed to stop compressor if pressure or temp is too low on low side and too high on high side. Compressor clutch can have a large gap to fight to pull in and be erratic if at the limit.
Odd ball problems include your voltage regulator - just a slight undervoltage in the car the clutch coil won't pull it but may have spikes where enough power is there and be intermittent.
Across the board about 65% of A/C problems are low refrigerant and this can cool or did so this problem is in the other 35% of possiblile problems.
It's really in the shop's hands now...... If the are stumped tell us what they're stuck on - Ex: No power getting to clutch coil or something,
T
Tom Greenleaf - MetroWest Boston - USA
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http://www.autoacsystems.com/tomgreenleaf/