I have a 2006 c5500 with the 6.6l duramax that has a p0087 dtc. in the beginning it was running rough and after checking it with the tech II found that the actual fuel pressure was not anywhere close to the demanded fuel rail pressure. I removed and replaced the high pressure fuel pump and the engine purrs like a new one now. the problem I have now is that when you accelerate over 2,000 rpm the truck goes into limp mode. if you stay under 2,000 the truck runs fine. the truck is not smoking like it was when it first came in. I have replaced the frpv and checked the lines to make sure that they are not collapsing under load. when the engine pedal is pushed to the floor the demanded fuel pressure is 20k to 23k psi but the actual psi never gets over 11k. does anyone have any other suggestions?