All great replies to the means of getting proper balance, which of course is definitly important. One thing I'm not sure of is the manner you are doing your roll. In order to do consecitve rolls it requires a tap of down elev each time inverted as well as tap of up when upright. Timing is the thing here. You can't just hold aileron and expect it to continue a roll without loseing speed and therefore altitude. Proper CG is not a cure all for everything each manuever requires proper input also. Maybe I'm barking up the wrong tree but since all replies centered on the CG just thought I'd throw that in.