The engine keeps revving fast then slow repeatedly every second or two.
While it still cuts the grass without problem it makes it's own mind up how
fast the engine goes, so I alternately walk slowly behind it then have to
run after it! The throttle setting has a noticeable, but small effect
sometimes, but only for a moment or two.

Suggestions?

The mower is a "Best Green" with a Briggs and Stratton 4 stroke engine.

David.