2000 F350 7.3L turbo runs bad at low RPM

[2K.F350]

I was driving along when the truck started running horrible, no power from start, shakes and rumbles upto about 2200 rpm then smooths out.
Took it to the shop the mechanic said it was the wiring harness. he replaced it, then said the other side is bad too. he replaced it. now he is saying that the # 4 and #8 cylinders are weak. wants to do a compression test.
any ideas? he is getting rich and i am getting poor quick.