Hi Jay... been a while
It could be momentarily too rich, fouling the sensor with soot and then better mixture cleans it up. Or it could be a big lean spot like you suspect. I'd say thats much more common. If it was fouling from too rich mixture, I would expect it to take a while to recover. Too lean would return to showing voltage instantly as the mixture falls back in line
The progression from idle jets to mains sounds too lean, a larger idle jet might help provide fuel closer to where the main jet starts taking over. Idle jets don't only affect idle. So.... maybe try a larger idle jet and set idle mixture with the screw back to where it was.
