I have an 02 Xterra with the Supercharger. When I start it up it has full HP. Step on the gas and she gets up and goes. After driving for about 15 minutes, all of a sudden, there's a significant loss of power. Step on the gas and it boggs down for a second and then gradually begins to pick up speed. The HP won't come back until it shut down and sits for a while. I took it to a local Nissan dealer and they ran tests for $100 and couldn't find anything wrong with it.Any idea what could be the cause of this? Could it be the Knock? Is there a difference between the Knock and the Knock Sensor?