What would cause the A/C to shut off and the A/C green light to flash?

What is your question?

Replaced the compressor 1 1/2 yrs. ago, along with the relay switch. Problem occurred in Aug. where it would do this intermittently, then it occurred more frequently, then A/C stopped. Mechanic spent a lot of time searching for cause. He replace the Pressure Switch and it worked fine for 30 days. Then the same problem began all over again. Did I need a pressure switch? What is causing this? What's the cost to repair?