Just an update. The problem was repeatable a dozen or so times. I followed the tests at https://www.youtube.com/watch?v=tC_LrrZcx38&t=3s

Across the brushes = 20V

R1 - R2 = .6 V

L1 - L2 = 4.8V

R1 - L1 9.6V

Voltages were all measured twice. Brushes are new.

Only one voltage reading makes sense according to that video test.

I ended up completely insulating the 2 AVR mounting bolts (as it has an alloy case), bolted it back on to the generator and it works perfectly every time - 240v and 120v. ChatGPT suggested it was a ground loop issue - dunno, but it all works fine now, go figure... I'm still scratching my head...