I have a 92 with h23 my problem is when I get up and going on the hwy my ass end sarts geting slopy

all my bushings seem to be good. I got more my speedo only works from 40 to 50 mph. one more question I want to go V TEC thats all I need is a h22 head , computer and V TEC harnes ? thanx for the help and any sugetions!!