You may be able to roll the tractor because someone put in auto relief valves. Someone will correct me if I'm wrong, but if you pull the tunnel cover off and the relief valves are flush on top they are auto relief. If there are little pins sticking out the top they are manual valves and may be sticking allowing you to roll it. Spray penetrant on the valves and work the pins to free them up.
When it speeds up and slows down, is the control lever moving also? If it is, there should be a nut in the dash tower you can snug up to keep it in place. If the handle is not moving you need to do some trunion repair.
http://cubfaq.com/trunionrepair.html
Bill