It looks like A/T does not hold the speed. 

 

You need to adjust your A/T Override setting in the PMDG SETUP menu to fit your flying style. In the takeoff roll, the A/T drops to HOLD mode, which allows the crew manually adjust (or close) the throttle. In the real aircraft, there are servos that match the physical throttle positions to where the A/T commands the throttle. Your hardware doesn't have this, so after hitting TO/GA, you should place your throttles at the forward stop.

 

I discuss it here:

Kyle thanks. I will take a look.