Short version: Right after my next door neighbor put in solar power, I began to have random light flickering issues. Both of our homes are fed off the same transformer (it is located between us). Is it possible that his equipment is "dirtying" up the power on the secondary side of the transformer?

Longer story: For context, my house (and Insteon switches) are five years old, and I never had a problem. Eight or nine months ago, when the flickering first started, I assumed it was something going on in my own house. I scoured this forum looking for similar issues and tried everything I could find here. I replaced the PLM, I disconnected/replaced UPS units, etc. I kept a notebook of everything I tried. Every branch circuit breaker in the house has been turned off at one point or another without any changes. So I know it most likely isn't any of my appliances or anything else internal to my house. I also watched the logs on the Admin Console when flickering would occur and nothing showed up there. Needless to say, it was maddening.

I never even considered that it could be the neighbor's solar set up until a couple of weeks ago. The solar company came out to replace some parts and had his system shut down for a few days, at least that's what the neighbor tells me. But for those few days, everything was rock solid in my house. As soon as they replaced the parts and started the system back up, the random flickering began again.

Is there some sort of whole-house filter I can install to potentially clean up the incoming power? Or any other suggestions of what I should try?

Take a look at this thread:

https://forum.universal-devices.com/topic/44344-issues-after-installing-tesla-powerwall-3/

Not sure why your house would be impacted by a neighbor doing something, but it might be a similar situation.

That Powerwall thread is the first thing that came to mind too. The powerline noise seems to be backfeeding through the pole transformer.