Rather than type it all out myself, these are the general causes of surging and hunting (which is what it sounds like your mower is doing) as copy pasted from google.
Common Causes and Solutions Blocked Carburetor Jet: The most common cause is a clogged carburetor, particularly the tiny pilot jet restricting fuel flow. Clean the carburetor, specifically the bowl nut and its jet, which are often to blame. Stale Fuel or Water: Fuel that has sat for too long can cause blockage, or water in the fuel tank can create erratic running. Failed Diaphragm: On some engines (especially older Briggs & Stratton), the carburetor diaphragm can harden or rip, causing air leaks and poor fuel flow. Air Leaks: A faulty or torn gasket between the carburetor and the engine intake can allow too much air, forcing the engine to hunt for a consistent RPM. Governor Spring Issue: A worn, damaged, or stretched carburetor spring can cause the governor to fail to stabilize the engine speed.
In my experience it usually either a dirty carb or a faulty or misconfigured spring (either throttle or governor).
This guy does some good videos on fixing mower issues.
If the above video doesn't help, look through his other videos you might find something useful.
