do you have a scoop grate? in my experience, scoop grates have had more of an affect on porpoising than the ride plate. learned this big time on my old sxi pro. had an extended worx ride plate shimmed down about 5/8 of an inch in the back and still couldn't get rid of the hopping. took the shims out, put a stock grate on, ran smooth as silk at any speed. just my 2 cents, of course, your results may vary.