Air Conditioner Problem

I have had both my air conditioners checked twice, the front air three times. They will run for about fifteen minutes and then the compressor turns off and comes back on and they cycle this way. The front air must be kept on high fan or it will freeze up every time. When the coach was new ,2005 Monaco La Palma, the air conditioners would run for hours without interruption. Now that front one runs only minutes after starting to cycle. The rear air does a little better. Running the front air at night with fan on high and thermostat on 72 just keeps the coach bearable. Like I said I have had the coach in to one of the largest RV sales and service garages in the midwest. They tell me the AC's are working fine but they are not. I would appreciate any input on my problem.