Q: a/c question

A/c button would flash 3x then go off... i figured out that means the system shut down the a/c. I added a can of refrig. and a/c work great... about 3 days later, same thing...3 flashes and nothing. Finally brought to my garage and they said leak. Added new o rings to the low and high valve caps... they charged the system. They ran for 40 minutes on the lot and the a/c was perfect. Brought home and next day, hot day, a/c didn't work...light lit up, but luke warm air conditioning... that night, started car and a/c worked tremendously. That symptom has carried on and off for 2 weeks now. My garaged checked the systme and all the numbers were fine. SO leak seems to be solved, BUT still inconsistnet a/c, cold or hot. ITs' funny cuz as i think this thru, when it works, the temps are not that hot out...on blazing hot days, doesn't work. Any ideas????

My car has 90000 miles.
My car has an automatic transmission.