Could be that the turnbuckle in the 3pt linkage is taken up too tight. When you raise it up all the way,do you hear the hydraulics "release" when it gets to the top or does it continue to labor a little?
Also,what do you know about the history of it? Did it have a loader on it before you bought it? Could be that somebody put the power beyond plug in and didn't take it out when they took the loader off,but if that was the case,it would lug down hard right from the time you started it up.