Roof mounted heat pumps

Mr Wizzard the compressor only runs for 2-3 minutes at a time then shuts off and the evaporator fan will run for a couple of seconds at a time for about 5 minutes then the compressor will come back on but for only 2-3 minutes. Thus when it is say 50 degrees outside the heat loss is as much as the heat gain thus it will never raise the inside temp. There are 4 sensors on this system and I think one in the plenum may be prematurely shutting the compressor off before the thermostat gets a chance to be satisfied. It is an electronic thermostat so it does not have an anticipator. If it had an anticipator I could adjust that but it does not. I do not think it has a high pressure cut out because if it did it would shut it off when it was in the A/C mode. I have asked Atwood for a wiring schematic but they say one does not exist. Which I think is BS.......A/C tech's would not be able to repair them with out one.....That may be why the repair place that installed it could not fix it.....