I am also having a very similar problem as well. When the car is cold my X starts first time and about an hour or so drive, it starts to slow down all of a sudden. Even if I rev the car upto 3000rpm, it still drives @ less than 20mph. Turn the car off and give it about 2 minutes or so, start it back up again and the car drives fine for another 10 before it misbehave again.

I'm wondering have you, PhazeX5 or anybody else in that matter rectify the problem yet.