I had the same problem with a 743 I had a few years ago, When I used a log splitter on it, when I deadheaded the log splitter it would raise the boob about 1/4 inch, after a few times of this you would have to get in the machine and lower the boom, I resealed the control valve but I did not change the springs, I just learned to live with it and it was still that way when I sold it.Sounds like a problem in the control valve. Weak centering spring or bad seal.