Hello, I just finished reading the 21 pages of this thread looking for a fix to this problem. I will give some background on my personal problem I am having with my 06 LBZ with 92,000 miles on it. When the following incident occurred, I already had the 4" down pipe back exhaust w/ no muffler, EGR block off plate, PPE race plug and PPE tuner on level 3, fuel rail pressure, boost and pyrometer gauges. This set up had been in the truck for years with no issues.

Last year 6/6/2015, i was traveling thru Mass. and stopped at a full service fuel station where, the kid pumping, filled my truck with gasoline. It was idling, I have 4" straight exhaust, a green gas cap that says diesel fuel only and I pulled up to the diesel pump. Needless to say, I only made it 3 miles up 495 before my truck died. I had no idea that it was full of gas. I cranked it over to try and start it, it tried to fire and the locked up. Hydro-locked, cylinders full of gas.
It got towed to GM certified Diesel repair shop #1, where they let it sit a week, fuel system full of gasoline, before they worked on it. They drained the fuel system, put some new diesel and an additive in it, changed the filter and said it was fixed. I drove out to Mass. (4 hours from my home) to get it, left the dealership, made it 150 miles down I-495 before my truck died. The only way I could get it to run was by pumping the manual primer, as soon as I quit priming it, it would quit.
Got towed to GM Certified Diesel repair center #2. There they replaced the CP3 and injectors. Called me a week later and said it was done. So again, I drive out to Mass. to get it. This time I make it home, and over the next few months it seems OK. So now its October and we decide to go camping, hook on my 30' camper and head up in the Adirondack Mountains. About an hour into the trip, yep, you guessed it, CEL comes on and the truck is in limp mode. First time this happened, did it 6 times on my 300 mile trip.
Over the winter, I battled with this issue by playing with the tunes, changed all rubber lines, replaced the filter housing, drained and cleaned the tank myself, cleaned the pickup in the tank, put in a PPE fuel rail fitting, took everything off and put the truck back to 100% stock, went back to my mods and nothing fixed the P0087 that pops up when I work my truck. It can be towing a trailer up a hill, hauling gravel in the bed, or just doing 4th gear brakestands, whenever i am hard on the throttle for extended periods of time, it will go in to limp mode and kick a P0087.
This spring I had to tow my camper to a motocross race, and drove 350 miles to and from the track in limp mode, and I continued to fight with the truck, but now it seems it will limp more easily. Used to be I had to really get on it to cause it to limp, but now I can be just driving the truck empty and if I go WOT long enough to pass a car or something, Voila, CEL comes on and P0087 is thrown. It now seems to be getting worse.
So I take it to GM Certified Diesel repair shop #3. They hook the gauges on to check low pressure supply rates and determine its a dirty fuel and air filter. Call me, say it's fixed. I go and get it and on my way home, I beat on it, and at about 125 mph, CEL come on and its in limp mode. Every time it goes it limp mode, at WOT my fuel rail pressure gauge goes from 26k at the initial throttle hit and fades down to 4-5k. Sometimes the truck shifts, and it goes back to 26k, sometimes it flashes up to 20k and bleeds back down to 4-5k then limp mode occurs.
I turned around and limped it back to the dealership, where try #4, they replace the CP3 again. I get it back, and limp mode again not 5 minutes after I picked it up. They want it back now to try replacing the ECM. I have had GM Customer Service involved now sing April of this year, and as of now,and they cannot figure it out. To sum it up, everything in the fuel system has been replaced, and now somethings twice. Any ideas on this one, truck was awesome before it got gasoline in the system. Its time to look elsewhere for the problem, so now they want to try replacing the ECM. I am going to suggest the fuel rail pressure sensor, sounds obvious to me, but what do I know. The truck all stock now except the exhaust and I can limp it by just hammering the throttle and not letting off until it does limp, generally by 85-90mph my fuel rail pressure gauge is dropping off and the CEL pops on.
Sorry for the lengthy post, but I have over a year trying to figure this out.