Have a 95 GP with a 3.1L, runs fine cruising around town, on the interstate (60+ MPH) at constant throttle it will shake. The shaking stops as soon as I let off the throttle, almost have to slowly pump the gas to keep it at 70. Replaced the throttle position sensor and does the same thing. Any ideas on what it might be? Plugs/Wires/Transmission? Car has 183k on it and don't know anything of the service history besides what I've done. Thanks!