prosine 2.0

I have been having an issue with the unit.
The unit is set up as a standby UPS system as follows:

Unit is connected to clean 120 VAC grid power.
Unit battery bank is about 400 amp hour sealed agm batteries (all good readings)
Unit provides backup power to computer/monitor/office light, etc. (Max load well within ratings) (heck, I have even run a 240 VAC well pump off this unit using a T240 transformer with no problems!)

Battery charger portion on float all the time. When grid power drops, inverter operates excellent.

When grid power returns, occasionally the unit does not return to charge mode, nor does the ac loads reconnect to the grid AC power.
The unit sits in standby...no charger, no inverter, no AC grid pass-thru.
Only by using the unit reset on/off switch to reboot the unit will it return the unit to normal operation. The display readout indicates low bus voltage, but a battery reading of 12.9 vdc is well within specs. I have checked for "bad AC" faults in memory, but no problems show up.

I can simulate the issue by turning the AC in breaker on and off a few times...it will not switch back to normal operation about 3-4 time out of 10.

I have called Xantrex, and although somewhat helpfull, they cannot offer suggestions other than sending the unit in for repair. It is out of warranty, so the cost of shipping both ways and who knows how much $$$ to repair it is scary.

They told me to check the usual suspects such as wiring etc.
They also mentioned it may be a software issue.

Anyone have any similar experience?
Any suggestions? thanks, jim