I have read various blogs on this topic. Lots of people suffer from this problem and it is very difficult to diagnose, even going through the diagnostics process. I did read one piece of advice and I have successfully appied it so will pass it on. I recently had restricted power several times over a few days which, touch wood, has not come back.
If you get the restricted performance warning light on and get restricted power, pull over and switch the ignition off, wait one minute, switch on the ignition but wait until all the lights on the dash have gone off except the four which stay on until the engine has been started, then start the car. In general, whenever starting the car, always wait until all the lights except the four have gone out. It is very tempting to start the car immediately, but it does appear that this could lead to the restricted performance issue.
Advice worked for me, so I pass it on.