Fridge problem

A friend of ours has a 150lt 2 door, 12 volt Danfoss compressor fridge (Vitrious or similar brand name) and is appears to be running correctly but then when the thermostat cuts the compressor it often only stops for a second or two, and sometimes 15 to 20 seconds and then restarts again. This causes the condensor inside the fridge to ice up and hence the temperature inside doesn't get cold enough. He has had service reps check it and they say there is nothing wrong with it, when there obviously is. Because it is running almost continuously, it causes problems with his batteries as the solar imput cannot keep up with power used by fridge. We turned it off, cleaned all the ice out of it, and then restarted it on a low thermostat setting but it didn't make any difference, it just ran continuously again. Has anyone got any ideas what the problem might be? Brian