It sounds like you are holding some up elevator during the roll. You need to concentrate on using only 1 control until you become proficient, then try adding in some down elevator.

This is the roll procedures for trainers, scale planes and full scale, non-aerobatic planes. I taught these same procedures as a T-37 instructor for the Air Force.

Line up parallel to the flight line at full power and a safe altitude.
Do the maneuver "by-the-numbers," one control at a time.
1. Raise the nose about 20 degrees and release all the up elevator.
2. Put in and hold full aileron. As the plane rolls, the nose will come down to level about the time you are inverted, then drop below the horizon as you finish the roll.
3. When the roll completes, release the ailerons and use up elevator to raise the nose back to level flight.

If you need to know other maneuvers, check the R/C Report web site for back issues of my column. I do a maneuver every month. Most are for the regular sport flier with a few 3D's thrown in. [link=http://www.rcreport.ws]R/C Report magazine[/link]