Almost every plane dives when it does a roll. The reason is that when the plane starts to roll, it looses the lift from the wings as the wings begin to tip. It continues to loose lift until it is flying on it's side, when there is virtually no lift. As the plane continues to roll to the inverted position it gains lift until it is completely inverted. As it continues to roll past the inverted position, it once again looses lift, etc.

To counteract this loss of lift and the consequent dive, there are a number of things that can be done.
1) Give up elevator just before you start the roll.
2) Increase the roll rate by making larger control surfaces.
3) Increase the roll rate by increasing the deflection of the control surfaces.
4) Increase the flow of air over the control surfaces by increasing the speed of the plane.
5) Increase the flow of air over the control surfaces by using a larger prop.

3D and 4D planes rely mostly on the thrust of their props to keep them in the air, rather than on the lift generated by the wings, so these planes do not "dive" nearly as much when they perform rolls.