I am wondering if you overheated the engine at one time.  I did so to an OS FL 70, and I think that ruined the head, and it lost much power, which I never could regain.  I have an OS 91 with pump too, that I have only flown 5 times, but I got it used.  It's running fine.  You might want to ask Bax at the OS Engines Support forum here on RCU for help and tips.  Could also be something wrong with the pump unit.  Good luck.  Jon