[MAlby7]

I know a bit about refrigeration because I completed my first year in HVAC @ Humber. If you A/C is going on and off it's because there's some sort of limit switch being thrown. This switch is being thrown after the A/C has been running for a while so it might have to do with the compressor starting to overheat because of overuse. This may be caused by either a blockage in the lines, not enough pressure in the lines, or possibly an incorrect charge of refrigerant.

On the other hand there might be a possibility that your thermostat might be malfunctioning and sending a signal to the A/C that the house has reached the desired temperature. Although unlikely this may be a possibility.

One thing that I could say for sure is that if you add more refrigerant make sure it is of the same chemical composition or you may cause a larger problem.