2000 6.0L runs rough and stumbles

[fxdxharley]

I have a 2000 silverado 6.0 with about 150k. just had the topend off to replace broken manifold bolts no where on cylinders!
It has been running rough for awhile now. under moderate load it stumbles and has a hesitation. I changed the plugs, fuel pump and filter about a yr ago. as well as the fuel pressure regulator and 2 o2 sen. No codes come up on the scanner so im at my wits end. anyone have simaliar probs? I dont want to have to pay a dealer threw the nose to figure it out.