Does it do the same thing in both directions?

If it's only in one direction then you've got your rudder and aileron trims crossed up so that one is compensating for the other being out of kilter or your wing has a warp and you are using the rudder to compensate. Any off balance trimming such as this can do what you describe but only in one direction. The other way should be fine or look like the opposite.

If it is the same to both sides then there are two things you can do. First is learn to use the rudder as rwlewis suggests and the second is that the vertical tail may be too small on this design. The tail sagging in the turns is often a sign of the vertical tail area being a bit small. This also shows up as a tendency to respond to adverse yawing during aileron use.

Another possibility is that your aileron linkages are set up in such a way that you have reverse differential to what is desireable. If your down traveling aileron defection angle is more than the upward deflection angle you will get adverse yawing despite having a lower aspect ratio than a glider. Often times the linkages are installed as convienient without realizing that you have to check for that sort of side effect.