Throttle problem

I am using the CH Flightsim Pro yoke. The throttle is linked to the Z axis and calibrates correctly. I am also using the latest version of FSUIPC with the throttle mapped, I think, correctly. The problem is that whenever I push the throttle up, it drops right back to idle by itself. Sometimes it does it almost at once without reaching full power; other times it will go to full power for 30 seconds and then drop back. Any ideas?