Omar,
I am in NO way a trim expert but if I may throw something out to think about until the experts give you their opinion...
If we are talking about power off downlinethen engine thrust would not have anything to do with it unless the up or down thrust is so gross you are having to put substantial elevator trim in for straight and level.
I believe pitiching up in the downline (power off) could be caused by wing incidence.
Have you checked it?
If you can adjust it try increasing the angle of attack of the wing slightly.
This will cause you to carry a little down trim on the elevator from where you are now. This in turn will decrease the pitch up, throttle off in the downline.
Make small changes!
Good luck!
JLK