There are some who would suggest the Leviton specials at Home Depot are crap, they absolutely are if they are 14-50 outlets. I'm not so sure about the 14-30 variety. I doubt it is causing your problem, though.
Can you try your neighbor's UMC, just to eliminate it as an issue?
247-248 volts is perfectly acceptable, dropping to 241 under load is also expected. Did you happen to check it while someone else is turning the microwave on? If the microwave is dragging one line particularly low, it might trigger some bad UMC behavior. It should be impossible for a microwave to draw enough power to really mess things up, though.