2004 GMC C2500 diesel hiccup

Engine Mechanical problem
2004 GMC C2500 V8 240000 miles

I have a chevy silverado with a 6.6L diesel that has a attitude problem.
When idleing in gear or in park it revs up every 5-10 seconds. It revs up hard enough that when I am in the Tim Hortons drive thru I have to hold the brake down with a little extra force or I am buying myself a new front bumper.
I have brought it to a mechanic and they installed new injector harnesses and injectors for # 2 and # 7 cylinders.
Also a new fuel pressure regulator was installed.
I have spent up to $ 2500.00 so far and still made no difference and am not in the mood to pay much more if there is no solid answer.
If you can help please let me know asap.