Problem with gauges (I guess) - Need help please

I would appreciate if anyone can help me with some new problem I have with my 2k2.Something strange happens with the auto throttle. I take off, get cruise altitude, and then cruise with no problem and the auto throttle works fine, but when I start descending the auto throttle gets stuck and doesn't matter if I set the speed at 350 kias or 200 or any speed, the auto throttle gets stuck and accelerates the plane to its maximum. I have to control the throttle manually, and the funniest is that at some point, when I am close to the runway, lets say 20 miles, I try it and it is working again.Can anyone help me with this? ThanksLeo