I was having some issues for a while which turned out to be the brake was sometimes dragging a bit, Just a little but it was enough to drop speed and stop the car from accelerating. Was mostly a problem in Forza as it does not have the handly little bar graph there in the hud to show your input levels. The problem ended up being a loose pot attached to the brake pedal.
Same as you but since I opened up the pedals and repositioned everything, all is ok now and has been for the last few hours.
No brake drag and I'm a bit faster now:-)
Red brake indicator stays off now when I'm not braking. It only needs a little drag to slow you right down.
Everything fine for now.
Cheers