Originally Posted by OTRtech
What engine ?
It could be just a bad pressure sensor.
If you have a 3.0 , it could be an indication of the cam syncro going bad.
X2 on the pressure sensor if it's happening upon start up. X2 on the cam syncro if it's happening while driving.
Replace the pressure sensor 1st, it's the cheapest and easiest. There's plenty of threads on it in here. If it's still happening, replace the cam syncro. Again, there's plenty of threads on it in here and also there's a great How-To as well in the Tech Section.