When setting up a flight, I set the CG to 30% in the FMC and I do not change the Trim setting from it's default setting (4 on the Elevator (Pitch) Trim & Position Indicator (-0.0^o)). Once the Autopilot is on, it takes control of the trim and it works fine for me.

When I have had the problem you speak of, it has only happened a few times, I have done a reinstall of the 767 Base Pack (using the repair option) and it works fine again for me. :-/