Does it feel like the throttle input stops as you approach the full throttle position? The throttle position is directly proportional to the torque output so does it feel like perhaps you're not getting 100% torque anymore? It takes a certain amount of torque to hold a certain speed, so I think if you go full throttle at 80mph and then roll off the throttle a tiny bit your speed should decrease right away, if you have to roll off the throttle a significant amount then there's a problem.
People have had a similar problem caused by a faulty throttle and I think on the older bikes it's easier to measure the resistance of them, a search on here for something like "throttle issue" will probably bring up some appropriate threads.
