Have they checked:
Lift pump pressure?
COV operation?
Rail pressure?
Injector return rates? (If high, DO NOT allow them to replace injectors without first re-torquing the cross-over tubes to see if this fixes the problem. More often than not they replace injectors and have to then torque the CO tubes leading one to think the inj. fixed it when really it was just the tubes).
Personally if it were mine I would torque the cross over tubes and see if that fixes it (very common). If you do it yourself it's just a matter of an hours labor.
