1999 f-250 7.3 power stroke . With straight single 3inch pipe. Power pup set at the 40 hp for towing k&n filter. The boost at a cruising speed drops to 0 on the gage and you can hear the wast gate open wile driving at a cruising speed. The truck has 165k miles on it. And it didn't start doing this until it got cold out. It won't hold its boost at any speed below 55mph.