Is the repair shop providing any insight as to the WHY you are experiencing this problem? It just doesn't make sense that three machines are all experiencing the same exact problem. It is quite possible that something you are doing is causing the issue. Do you wash or even worse power-wash your units, inadvertently introducing water/moisture to the electrical components? Are you engaging the PTO at the right throttle? ( I know this sounds silly but my eXmark specifically requires engagement at mid-throttle) Is the voltage regulator operating properly and is it grounded directly to the frame? Has the entire electrical harness on these machines been inspected for shorts or missing casing on wires? Just some thoughts. I hope you get this figured out as electrical problems are some of the worst to deal with on anything you own!
Mike