hi there reefer C, have you checked the TX valve. I had one not to long ago that would do the same. We narrowed it down to the TX Valve. It will go into defrost if it detects there is no difference in supply and return
e.g evaporator coil iced up. Try taking it off auto defrost and running it you will see what the suction pressure is doing. The TX valve might be closing prematurely, this can be caused by moisture in the system, have yiou tried a new filter drier and refrigerant charge

Try these and see where you go

good luck