P0191 06 2500 LBZ, HELP.
 

I have an 06 Duramax with a P0191 code and goes into limp mode intermittenly while drivng. P0191 isnt a code set while the vehicle is running, Ive tested the 5V reference circuit and its good, but like I said its intermitten. Vehicle runs good until limp mode. No power loss till then, the FRP sensor and the Fuel Pressure Regulator on the pump have been replaced. Tested continuity of all circuits for FRP sensor and no shorts per Alldata. Could the High Pressure pump be bad and not set codes, like a lack of fuel volume? No aftermarket mods or programming. 102k miles. Brand new fuel filter. Where is the check valve for bleed down of the fuel rails on this system? Is it in the pump, or is that the Regulator on the pump?
Any ideas or has someone run into this before?
thank you.
Eric