This is a Dyna, not a softail where the oil tank is above the crank case; of course gravity will drain the oil into the sump. Dyna and touring models return oil from the crank case and cam chest via the scavenge side of the pump that pulls oil from both and pushes it through the return passages to the tank. If at any time, the oil tank is low or empty, the oil has to be in the crank case sump or the cam chest. Why it is an "on/off" situation, I can't answer but it would appear that the scavenge side of the pump is not functioning properly an that's where the OP should start.

It would also be a good idea to hook up a pressure gauge to the sending unit port and check oil pressure first. JMHO.