bizarre throttle problems

I'm experiencing some really strange throttle problems with the PMDG 737-800. Today, while enroute from MCT to DXB, the plane jumped from 280 KIAS to 700+ KIAS in a split second and crashed from overstress. When I reloaded the flight and had the same thing happen, I started the flight over, only to have the plane jump into the 700+ KIAS range as soon as I engaged TA/GA mode. At this point, I restarted the computer, set up a new flight, and this time, the thrust reversers engaged and sent the plane in backwards circles at very high speed (didn't notice the airspeed indicator). I've never had a problem with any of my PMDG 737s before, so I'm hoping someone here can shed some light on what might be causing this. Thanks!