No, inverted isn't the problem, but something else is. It sounds to me like you have it too lean on the high-speed needle. The ability to run OK at low throttle settings, but not at full power, suggests a lean condition, or overheating (which could also come from being too lean). Did you ever overheat it in the past -- this can also cause a failure like the one that you described. Another possibility that I hesitate to mention is a peeled liner. Or it could be as simple as something may be wonky in your fuel delivery system (tank, vent, fuel lines).

Good luck