Before making a major decision about a different control system, you could try operating it on 60Hz and see if that is the problem. If it is, you can figure out what else to do. You do not have to "run a 60Hz inverter continuously," only long enough to determine if that is the problem or not. If 50Hz is not the problem you can look elsewhere.
Since no one here (so far) can tell you definitively what the problem is, you will have to do some more investigation. It's better to make some tests that point you in the right direction and eliminate phantoms than it is to just speculate and make knee jerk decisions based on someone else's guesses and speculation.
