914 RPM drop on take off

Has anyone experienced this who can steer me in the right direction?

I have a Rotax 914 which is losing power on take off. The problem has occurred in my last two flights, both times at 200 feet at just under full power. RPM dropped rapidly from 5000 RPM back to 4500 then power returned and then RPM dropped to 4200. There was no rough running or other warning signs before the drop in RPM and the drop was instant not gradual, as if the engine had lost all power and when it regains power the RPM stabilized at whatever it had dropped to. I had a gopro running the last time and it showed no instruments indicating any issues. Oil and fuel pressure, quantity and temps all good. Both times I was able to return to the airfield with sufficient power at 4000 RPM. I have had issues with the turbo over boosting sometimes and other times taking a long time to generate any boost. The engine is 20 years old with 1100 hours. It has other issues - showing early signs of “fretting” - hard to turn the prop by hand when the engine is hot, indicating a compressed crankshaft as I understand. But that seems irrelevant given the engine would not have been hot enough on take off to compress the crankshaft and it was otherwise running fine. There is also a blinking yellow light on startup indicating a TCU failure but the light goes out if I restart the TCU by pulling the fuse and then reinserting it. I am wondering if I should keep pouring time and money into this engine or bite the bullet and buy a new one. Any comments welcome.