Change the fuel filter and if it still does it then start looking for air in the fuel lines possibly caused by tank screen getting blocked or just air ingress somewhere along the lines and joints in the system.
A blocked fuel filter will give the kangarooing but the tell tale here is to run it at a constant engine speed until it does it and then back off about 300 revs and it will behave itself and act normal until you increase the revs again. Another symptom of fuel starvation is white / grey smoke accompanying the surging ie not enough fuel to burn.
I have had a few partially blocked filters over the years because I was using veg oil and each time the symptoms were first spotted when on a long drive and using say 3k revs for over 20 minutes or more and then you pick up on the gentle slow up as it starts to starve of fuel so back off and it goes away and increase slowly and back it comes.
I found the om603 would tolerate a bad filter where the om606 would happily start drawing air in everywhere with its o ring fittings .