1. When did this problem start? After maintenance? Suddenly? Gradually?
2. Bleed air leak, cracked discharge tube, cracked combustion can? I'm no Twin Star mechanic, does #2 provide a customer bleed? If so, cap it for testing. As above, cap the anti-ice too. Biggest culprit of high
Ultimately, if it passes a power check, then you can keep running that engine but if you're exhausted all your other options, I'd hazard a guess that your turbine assembly is damaged.
Make sure you report back with your findings!