Autopilot

Hi, I am using the 737-500 by FFX and the aircraft is fine except when i engage the autothrottle switch on the autopilot, it then starts to rev the engines/gauges and throttle levers up and down, if i hand fly it the speed its fine ,any help to fix this if would be appreciated. If i use the PMDG 737 with the same panel as in the FFX model it does not happen, so is there something i can change/add to the FFX aircraft CFG file . Thanks.