Some things that helped me with that problem...

Check the wing. Make sure it is tight, straight.

Check the tail assembly, the fin that is parallel to the ground can get loose and move back and forth. Jam something between the fuselage and the fin to keep it stiff(piece of wood, or hot-glue).

Make sure the servos are well-glued in, if not hot-glue or epoxy them in.

Take off the cowl and check the motor mount(the plastic part on mine was loose causing erratic flying). Hot glue that sucker so that it stays put. This solved a lot of problems on mine because it was loose the plane kept going right.

Check your prop, is it straight, bent, dinged?

You may have to retrim the servos by hand (I don't know the techy term for it). Unscrew the little white arms when the receiver is turned off. Turn on the receive with the trim at neutral. Manually make the rudder/elevator straight, then carefully attach the white arms again to the servo, trying to keep them exactly straight. The plane should now be trimmed in neutral. You will still have to adjust it though.

Hope one of those works, but mostly a big crash can jar stuff loose that you don't see but can make the plane fly funny. I had to completely re-assemble mine after a particularly hard crash which looked innocent, but in the end had loosened the servos and the motor mount, making it very hard to fly straight.

Have fun!
/Tom