I started to have problems with the a/c. It did not work so I took it to the local shop and they replaced a relay and reseviced the unit. It works but now it will stop putting out cold air for about 10 minutes then go back to working again. Went back to the shop, they checked it but could not find anything wrong with it. They said it was a little low so they serviced it. After driving it again the problem came back,cycling off then back on . What could the problem be?