Intermittent uncontrollable excess movement of cursor & extreme magnification of browser windows

Does anyone have a solution to these two linked problems? I've spoke with three different Apple Tech support people, tried a new wifi Magic Mouse (no better than my 2014 USB-connected mouse), tried SMC. Nothing works. The problems occur while I'm in Safari and Firefox and using the mouse on my Desktop but not in Adobe Creative Suite. They are intermittent but getting more and more frequent. It's probably coincidental but both problems have stopped since I signed into the Apple Support Communities.