First thanks for the reply Pete.
I had initially calibrated and assigned the entire Saitek Throttle Assembly (Throttle, Mixture, and Prop) through FSUIPC. They worked flawlessly for about two months.
I ensured that these controls did not appear in the FSX Settings, as I had read about the conflicts that can occur when they appear in FSX Settings also.
When the erratic Mixture problem occurred, as I stated, I first suspected a dirty pot on the Mixture control but found that not to be the case. I also again checked the FSX Settings to verify that they did not appear there. There had not been any hardware changes during this period, and the only software changes have been scenery installations.
To rectify the issue, I removed the FSUIPC assignment of the three axis and assigned them through the FSX Settings. They have been operating properly since I did this, and I have not tried reverting to a FSUIPC assignment (the old "if it ain't broke" saying).
When I have time, I will attempt to reassign them through FSUIPC and report the results.
Again, thanks for the response and info. Don't know what I'd do without FSUIPC for all other functions.
RodeoGeorge