Porsche911 I keep it simple and to the point I have a os91fx and it runs perfect several hundred flights and no dead sticks, But I have had the same exact problems on a Supertiger 51 ringed It's scary how you threads and my experence with the motor was similar.

After all the problems I faced it turned out the ST HAD A BROKEN RING SHEEEESH!!!!! HAD 40 DEADSTICKS WITH THAT ONE just like you the needle never seemed to be set right and it changed on a min by min basis.

I think it may be time to pull the piston on that 91 and check the rings.

Hope this helps