Follow up......

Completely disassembled both extruders.
Found very little debris in the gears but blew them out. Only a litle dust so I assume that was not the problem.

Removed the nozzles. One appeared clogged. Took a torch to both of them and heated them until everything in or on them turned to a fine ash powder.
Cleaned them both thoroughly afterwards. Looked through them....both now clear.

One PTFE tube had a bit of hardened debris in it. Cleaned both PTFE tubes thoroughly with a 2mm drill bit shaft.

Ordered new PTFE tubing but it wont arrive for about 10 days so reassembled the assembly and running a test now.

So far so good. Print looks great....but the problem before was not at the beginning of a print, but rather about 2 - 4 hours in.

I have the printer on a battery backup so short power outages don't stop the printing. It is running from the internal SD card slot...not from an external computer.

If the problem persists, I will assume there may be some kind of intermittent power supply issue and open up the power supply and if I find nothing, probably look for a replacement. Don't know what else it could be.

One odd clue to this problem is that when the problem occurs, it does not seem to continue spewing filament. It seems to stop flowing filament altogether. There is not a mess of tangled filament. Instead, it looks like the filament flow just tapered off and left a slight "fluffy" appearance to the top of the print as the filament tapered off to zero flow.

My best WAG is that for some reason, the heaters turned off at some point during the print.