The APM20 that I use, in modified form, does 130 kias, at my usual cruise settings. In FSX, on a couple of occasions, it's considerably exceeded this, on the same settings. At the time it's bee held at 3000', by the Auto Pilot. The increased speeds have been accompanied by a definite nose down attitude; confirmed by looking via Spot Plane view.
Could it be that FSX has had significant upcurrents in the areas I was passing through?