Had the same problem with Ultra Sport 1000. On take -off the plane would start a very slow motion roll to the right and I couldn't figure it out.
One day it struct home that the total weight of the ST 90 was to the right of the longitudinal axis. So out to the balancing hangar. Using a number of nuts and bolts I determined the exact amount of extra weight that was needed to the left wing tip. I found a small rod 5 inches long equal to the weight needed and shoved it into a small hole drilled front the front into the wing tip. I resealed the monokote and the hole disappeared hiding the small rod foreever.
This plane flew so well that full throttle with a click of up elevator and the plane would take off alone...once with the transmitter on the ground...and flew straight as an arrow.
Good luck