codimasta,
I'm curious, what do you mean by "porpoise"?

Do you mean at different throttle settings it either wants to climb or descend?
Maybe your thrust angle is pointing up or down.

Do you mean with the same throttle setting it either wants to climb or descend depending on the speed?
Maybe you are nose or tail heavy and you are trying to compensate with elevator trim. That's okay for a given speed but if you raise or lower your speed, the trim setting will have more or less of an effect respectively.
Or Maybe you're trying to compensate for an incorrect incidence angle at the wing or stab by using elevator trim.

Good luck!!