742F Autothrottle Issues

I'm having an issue with the AutoThrottle on the 747-200F, If I activate it and it runs for any length of time, then when I try to shut it off, it stays active, even with the switches off.......Twice now I've had to land with 2 engines shut down and the spoilers up to get it slowed enough to touch down..... However, when I do land the Reversers will deploy and function. So the Throttle itself is reacting to my inputs, but the Thrust level isn't..... Any help please...