That problem can be caused by almost any trim misalighment, I just trimmed my new model and it was terrible in level before I cleaned all trim issues. Now it goes like on rails and I did not need to concentrate to just "that". It leads to that prop choice. Many flyers already found that pusher is not fully trimable prop on classic models (wing and landing gears under the thrust line), so my recomendation (if mentioned friction problesm are solved) is to use normal prop and concentrate to trim the model. Then you will see if there is still that problem or not.
However there are still few tricks which SOMETIMES help in this case.
Try motor downthrust ~1 deg (if it does not hurt figures)
If you have front line up, try to exchange them in LO guide, front line down stabilizes model (WITH TRACTOR PROP), and keep line spacing ~1" or even more.
If you have moderate stab LE diameter, try turbulators on stab, or try to deflect elevator little bit down. (if it does not hurt figures)
If you do any of those changes or if you change the prop, and if you have PW chard front of you, you will see that before judging whether it helps or not, you will need completaly retrim the model, only then you can say if it helps or not.
EDIT: and do not forget that IC model has tank with fuel which moves CG forward as mentioned in other posts, so if you have CG as per IC plan, you have it tail heavy