If the power required exceeds the specifications of the device,
presumably the field has to fail in some manner.  Someone has to
decide whether that would be as uneventful as the object gradually
regaining some of its weight, or as catastrophic as sudden field
collapse dumping all its energy into the grav plates and turning much
of the vehicle around them into flaming wreckage.

I tend to favour the former, as contragrav is portrayed in Traveller
as being very reliable.  It also fits in with the idea of stored field
energy being "drained" into gravitational potential energy as the
object rises, and having to be replenished if the object is to remain
fully shielded from gravity during the ascent.  It's stable in the
sense that failure to provide enough power is self-correcting: the
returning weight of the vehicle from a weakening field would generally
slow the ascent and reduce the power requirement.