I will take a guess. The initial warm up is used to charge a capacitor to the point where it will activate the relay. (There is more to it than that but that is the basics) If that cap is going bad it will leak voiltage when the unit is on and current is being drawn through it. The voltage being supplied to the relay will drop below what is needed to activate the relay (turning the unit off). When the relay opens current isn't flowing allowing the cap to recharge again reaching the specified voltage (turning the unit back on). Then the process repeats itself. This can be a dificult problem to trouble shoot. A physical inspection of the capacitiors may reveal a leaky cap. If not test equipment will be required as there may be more than one cap. It could also be something else but this is a starting point.
Remember to take the system as a whole.
Remember to take the system as a whole.