AC 15000 Low Profile Dometic Penguin

Recently had a Dometic 15000 Btu low profile AC installed replacing a 13000. After unit runs for a period of time, from one to four hours, the fan starts cycling. By cycling, it changes speed from high to low, low to high or sometimes just off and on at the what appears to be the same speed. These cycles occur on one minute internals and will not stop until the fan is taken off Auto and put on High speed. The thermostat has been changed twice as well as the circuit board on the unit. Anyone having this problem or having a solution?