I have an 04 350 diesel. Runs smooth, plenty of power, decent fuel milage, but at a steady speed it will surge and the tac will jump about
500 rpms. It will give off a puff of black smoke. If stopped when this happens, it will generally stall the motor, or it will idle very rough until you put it in nutural and rev the motor, then it will smooth out and run fine again.
Things I have done to try to solve this:
replaced fuel filters, egr valve (it was leaking and full of soot), flushed the system completely out, also flushed out intake and injectors. I ran two tanks of fuel with Lucas Injector cleaner and diesel fuel conditioner.

 

additional information:
I am thinking about changing the throttle postitioning sensor. Does any one know what this involves, or the cost? Any Ideas??????? Desperate!!!!!!!!