Ok, I got beat to continuity checks (
).
I don't think you'll find a common wiper, but it wont hurt to check the worm grease while you are this deep. They really don't differ much visually from post war GG-1s or Williams so I think the bushings and frame do a good enough job at power delivery. I've only seen loco wheel/axle wipers for common on 2r and other gauges. Cars are another story. I think they only have them because of delrin bearings.
If the board draws com. from the chassis and there is no truck to chassis wire, see if you can add one. If the board draws common from trucks I doubt there is an issue there and I'd suspect the roller wire. Andwwe missed clean the roller and center rail. My center collects more crap than outers and wheels...used to be the other way around on the floor. (Not looks clean, clean them. And though I've never had issue on cruddy track, Popi is seldom off base, clean the wheels and track again..for him. If he lived within an hour, he'd likely show up and do it for you 
)
When you oil axles, are you getting black drips? If so you should flush them till it runs clean. Black is metal and/or dirt. (flush with multiple oilings. or WD-40, crc electric cleaner, etc.. then when dry, re-oil.)
Do you have a (kinda big) DC power supply or bridge rectifier you could use? Bypassing the board with dc could eliminate or confirm that board as the issue (a 9v bat. might work..just not long. Is the Bug 6volt? I've run trains in my Kombi off it's bat., lol )
(And because of 1st snow on the ground and your timing, I'm thinking about my first Bug never getting stuck for more than 10min. It would have to run up a pile and get stuck ON the snow, 4wheels inithe air. There, hot engine air would blow-melt the snow at the rear allowing me to back up and plow past with some extra speed I also drove the buggies in it and on lake ice when I could 
)
