On all A-B drives, the drive does not directly monitor the incoming power, it only monitors the EFFECT that the incoming power has on the DC bus. So all A-B drives are inherently capable of being fed with single phase power and putting out 3 phase, so long as it has enough DC bus capacitance to smooth out the added ripple caused by single phase.
Which brings us to your issue. You EITHER have an intermittent phase loss taking place, OR, you have a bad DC bus capacitor inside of the VFD. If it's a bad cap, you can get a new Power Module for that drive. I don't think the bus caps are easily replaceable and besides, if you lost a DC bus cap, something caused it and that likely caused other damage.
A common cause of issues with the DC bus caps is if you have a contactor ahead o the VFD that opens and closes a lot. The VFD has what's called a "pre-charge resistor" that prevents the DC bus caps from drawing too much current too fast when first energized, which can damage them. The resistor in only in the circuit for a second when you first energize the drive, then it is shunted out. If you are constantly opening and closing a contactor ahead of the drive, that resistor burns out and then the capacitors start failing. If you do have a line side contactor, re-think why it is there.