Is the unloader a K-7? Mine was doing the same thing and we corrected it by changing the size of the orifice to the bypass line. What size hose is your bypass line?
OK, whether it's a K-1, K-7 or K-10, it's a flow-actuated unloader. Sometimes a larger bypass hose will cure the problem, other times the orifice needs to be reduced to a smaller one.
To quickly trouble shoot this is what I did... MIne was surging... I put a hose clamp around the unloader hose and slowly tightened it up... It stopped surging after a few twist on the clamp... I ran the machine like that until the smaller unloader orifice came in.\