I think I may have stumbled upon a possible solution to your problem! My LHS was telling me a story about this engine. He said there was a history of the needle vavles turning on their own when certain vibrations were in the engine. What I would do is tune the engine for good response and shut it down. Now mark the needle valves somehow, with nailpolish, to give what is termed a "witness mark". Make sure you mark both the needle and a "fixed" spot also. Go fly until the aircraft does this again. Once it does it again land it and verify that the needles are not vibrating to a different setting. I would try to do this on high, low, and air bleed if possible! Hope this helps. He told me a similar story and I remembered this post!!
Nik