What could cause this?

Before anyone asks I plan on having an electrician come out soon. I have also had the utility come out in the past and said everything is good on their end.

This has been an on and off issue since I bought my home a few years ago. It is currently happening again. When there is a large appliance running (especially the furnace) it will flicker / dim all the lights throughout the house. It being inconsistent is part of what makes me so confused. I can have a day where it doesn't do anything at all even when several appliances are on or it will happen bad when just 1 is on. We will go like 95% of the year and it wont happen at all and then a few weeks where it's bad. It happens when the drier runs too but not nearly as bad. We've always had some flickering when things like the keuring or microwave are going but those appear to be localized to that circuit (from what I can tell). I went and bought a $7 outlet tester and tried every outlet I could find and never got a hot ground reverse (from what YT video I watched that was what I was suppose to be looking for) I did find a few with open grounds but my house is over 100 years old so it doesn't surprise me that some of the outlets are outdated. From what I've read it is most likely a loose neutral inside my breaker box or meter box. I had an electrician in my breaker box a few years ago and he couldn't find anything wrong either. I'm open to hear all opinions from yall. What I can't understand is the inconsistency in which it happens and why no one can find what it is.