OK, two things you need to do.

First of all your elevator is way too wide. Cut it down to about 1/3 the width it is.

Second, the ailerons should probably be a little narrower and go out much futher. Make them long enough to reach the part where the wing starts to curve up to the tips.

Note how the wings are drooped? That is going to tend to make the model sideslip like you described. Adding up elevator a little sooner in the turns will help a lot. I suspect you roll the model and then apply elevator after. You need to blend the elevator control with the roll maneuver better and that will help prevent the side slipping. If it's hard to do then try adding some dihedral so the wings are angled upwards instead of drooping.