Sounds like a downthrust issue to me. If you cut throttle while flying upright straight & level, does the plane rise? And does it descend when adding throttle flying upright straight & level? These are symptoms of too much downthrust.

With a side-mounted engine on profile rail mounts, I guess the cure would be to remove the engine, degrease the mount blocks with alcohol, let dry, then plug the mount holes with dowels, epoxy them in, let cure, then drill new mount holes. If you butcher that part (like I sometimes do) then the next cure is to cut off the nose and install a firewall and then go to radial mount for engine, which will greatly facilitate manipulating engine thrust offset.

Next question is how much correction in downthrust to make. It takes a trained eye to get this right with the first guess. You need someone with the trained eye to watch it fly, or fly it himself and then give you his estimate. If you have to take a stab at it, try just a couple degrees. If you really want it straightened out for any throttle action, the only way to get it just right is test flight, adjust, test again, etc. If you want it that good, you might as well go straight to a firewall installation and take it from there.