Burning electrical smell from furnace

I've been dealing with this issue on and off for years, and I can't seem to find the cause. This all started when I replaced the thermostat years ago, and it has continued even with new thermostats.

There is a very strong burning electrical smell (ozone, I believe) coming from the furnace intermittently. Sometimes I can use it for months and never smell it; sometimes, it only happens occasionally, and right now, it's super strong. It seems to be the worst when the furnace has not been used for a while. No, it's not the dusty metallic smell you get when the furnace hasn't been used; this is most definitely electrical; I've shorted enough wires working on cars that I know it well.

A couple of years back, I pulled the furnace and replaced every single component, circuit board, safety switches, etc. The only thing I didn't replace was the blower motor. So this year, I thought surely it has to be the motor, so I replaced that as well. It seemed to help at first, but that could have been coincidental.

I have inspected all the wires and have found zero signs of shorts or melted wires. Where do I go from here? Thanks!