Trace SW4024 refuses to stay sync'd with grid AC and charge

Been living with an older (circa 1999) SW4024 on a grid tied system with PVs. Just recently we were under a forced evacuation due to a nearby fire, grid AC was turned off. I made sure to turn off all non-essential loads on the PV before I left, but expected that they would still slightly overdraw the system. We were gone for 4 days. We just returned, no damage to anything. The AE system was pretty much as I expected, the sw4024 had shut itself off due to low batt (24.0v), solar during the day appears to have raised the batt voltage to 24.5v but that was still less than the cut-in voltage of 25v. No problem, just waited for the grid to go back on.

When the grid did finally come back, the inverter went into a strange cycle. Tie light is yellow. When grid is applied to AC1 the AC1 good light flashes green. If the inverter is switched off, it NEVER goes solid green indicating good AC, just keeps flashing. If the inverter is on, it flashes for 15 sec or so and then it goes solid. The bulk charge light comes on solid yellow (as expected, the batt is low). After about 10 sec of charging the bulk light goes off and the AC1 good light starts flashing again. While the inverter is on, this cycle can repeat for many hours.

I have tried turning the AC1 input off for a while (at its breaker). Does not make any diff. I have tried removing almost all the AC loads from the inverter, again no difference. FWIW, when I watch the batt voltage and current during that 10 sec interval when it is supposedly doing its bulk charge, I see absolutely no difference in voltage or current on the batts. Does not seem that bulk charging is really ever starting in spite of what the light may say.

When I monitor the grid AC with a DVM, I do not see any obvious dips or blips at all, and nothing that would sync up with the sw4024 cycling. Could be frequency or waveform variations I suppose, but unfortunately I don't have a way to measure those. I did check with some neighbors who have grid tied inverters and their equipment is not having any problems (not older trace stuff, but still).

This inverter has been rock solid for almost 20 years. It has gone through many times of the grid going down, inverting from the batts, and then coming back up and recharging the batts when the grid is back; most recently about 6 weeks ago. Although most (but not all) of those times the batts did not get drawn down to low cut-off voltage.

Right now, my next step is going to be to wait for a sunny day to get the batts up over the cut-in voltage and see if that makes any difference.

Other than that, thought I would ask here, to see if someone out there can see what might be going on from these symptoms. Anyone know what the trace checks at the beginning of a bulk charge, and what (if anything) might kick it back out of bulk and have it start rechecking the quality of the incoming AC?

Any ideas?

Thanks in advance.