Chuck is using Firefox and recently he's been getting error messages. Leo says that may be due to keeping a lot of tabs open. Should he go back to Safari? Leo says that it's about as good, privacy-wise, as Firefox. But for search, he's currently using StartPage.com for searching anonymously. Use that, and it's just as private as Firefox.
His iMac screen is starting to flash black and white after a few hours. Leo says it's more likely a hardware issue, but you should definitely take it to the Apple Store. If it's overheating, then there's some obstruction with the air cooling. Apple is going to have to clean it out. It would be very difficult to clean it out by hand. One thing to try is a clean install of macOS. If that doesn't fix it, Apple will at least have a clean canvas to work from. He can also download iStat Menu and see how it is overheating.