Check the horizontal stabilizer to wing alignment; ie they are parallel, a visual should confirm that. If one wing tip is higher, then inserting some wing seating tape on that side of the wing seat will help.

When attaching the horizontal stabilizer to the fuselage in any plane where the stabilizer cutout is precut from the manufacturer, it is important that the stabilizer is attached with the wing fully seated and bolted down on the fuselage to check for elevator alignment. Even a few degrees out can have an exagerrated effect on perfomance. I found this the case with most similarly constructed arfs.