Could also be the governor spring which automatically controls engine rpm. Under load, the governor would rev up engine to keep at optimum cutting rpm. If spring is weak, bent, or mis-adjusted, surging can happen. It is possible to adjust spring, but to do correctly, will need some way to measure rpm, and not to meby, the recommended operating rpm. Might have to replace spring if it is damaged, that is saying it is the spring. I agree with a tune up first. Plug, air filter, and fresh fuel.