Even though you've cleaned the carbs several times, it's easy to overlook a small port that could still be clogged. When you cleaned the carbs, did you use carb cleaner and then blow everything clean with compressed air? If you have strong, steady current going to the plugs and electrical system otherwise OK, then it pretty well has to be a carb or fuel delivery system issue. When you sprayed ether into the carbs, it would have increased the RPMs and perhaps given you the impression that it smoothed out. Also a worn crankshaft bearing can cause a crankshaft wobble that generally shows up as rough idle and goes away at higher RPMs.