Lately, after extended highway drives (2+ hours on the highway) the Uplander will bang then the RPM's shoot up, when this happens if you let off the gas the RPM will drop to idle speed. It will go on with high RPM for about 2 minutes or so then will finally shift back down to normal RPM on the highway. About every 15-30 minutes it will do this. Well it finally threw a CEL so I just went down to Advanced and I had it read with an OBD II scanner. It gave me code PO716 "Turbine input shaft speed sensor". What could this be?