UPSs are available in many hundreds of sizes and configurations, including whole house units designed to compensate for brownouts.

I like Cyberpower units. I think using one would solve the problem but I would send them an email first and see what they think.
Here is one that I think is in the ballpark.
https://www.cyberpowersystems.com/products/ups/avr/avrg750u

Data logging is a feature that will help you here. You'll be able to see the voltage over a specific period of time.

Seems like logging the voltage coming to the flipper might be beneficial. The game may compensate for small variations from the wall.

A drop of 0.83% ( zero point eight three percent ) should fall well within the design range of anything plugged into your outlet. A five percent tolerance is normal, ten is frequently acceptable.

Your testing methodology isn't providing you with any data that's going to help solve your problem.

You need to know if the power at the coil in question is low when the problem is happening. A data logging multi meter can tell you that.
https://www.amazon.com/Ideal-Industries-61-498-Datalogging-Multimeter/dp/B008SEPOCO