I havnt myself Martin, but it's interesting what you said regarding projectors not being tested with polyester. Bill Parsons and Kevin Brown mentioned this to me a few years ago. When i had a problem on one of my 16mm projectors where the gate noise was very loud, i was told then that all our projectors were made for, and adjusted for the common film stocks of the day. In the later years when the thin polyester films came about, adjusting the pressure plate/gates became a more difficult task. I know the projector runs very quite through the gate with polyester films, but when i had a problem with my SL2 after messing about with the gate myself, Kevin Brown sorted it and said, you can adjust it to run fine with older acetate stock, then run a polyester print and the film jumps, its a critical adjustment to make sure all the films run fine, even then, you may find the odd film may still want to jump, especially ones that have been over cleaned.
I guess with 8mm it becomes even more of a fine adjustment, and as parts wear, perhaps the thin stock films begin to either jump up and down, (as one of my 1200HD's did) or the focus becomes unstable. My 1200HD required a replacement backplate after some films became unsteady on screen, but it had done 40 odd years of projecting movies.
Hopefully one of our Eumig users will have an answer for you, otherwise, John White may be the best port of call. Let us know how you get on please.