There are a number of issues that can cause that.... Bad FICM, high-pressure oil leak, bad HPOP, debris in the injectors due to an oil sump screen, broken wire in the engine harness, bad ICP sensor or solenoid, faulty oil pump...
You need to just go a dealer, or an independent shop with the proper 6.0 diagnostic equipment. If the shop starts suggesting swapping parts, tow it somewhere else. When the diagnostics is done the bad part is identified, be sure to ask about the cost of repairing any consequential damage (for example, a bad FICM can damage all of the injectors, which cost over $3,000 for the set) so you can decide up front if its worth fixing.