I always learn something new here. Thanks for the explanation. I actually understood most of that.
Looks like I was wrong. The furnace, washer, and dryer all share that 20A circuit. I've never noticed it before, but the washer and dryer also cause a bit of a surge when they start up; just not as severe as when the blower motor starts. As luck would have it, the living room seems to be the most affected area (where the lights brighten up most), and that's where the TV and stereo stuff is.
Belt driven, wow that's gotta be old. On fuses or breakers? Newer houses is usually code for a furnace to be on its own breaker/circuit. If your furnace is belt driven, it could mean that is 30+ yrs old, and if you house is fuses, there very well could be 20 things on a circuit.
Built in 1978. It uses 15, 20, and 50A breakers.
1970s furnace? I wouldn't bother upgrading or retrofitting it. Just keep getting the heat exchanger inspected and save for the replacement.
In the meantime, a "floating neutral" condition sounds like the best explanation and likely indicates a significant problem forming at either the box or the transformer neutral connection. DO have it inspected soon.
None of the other appliances in the house produce voltage spikes. It just seems to be the one 20A breaker that's acting up. Is this still a "floating neutral?"