The main reason behind this surging could be an unclean carburetor, clean all parts of the carburetor with compressed air and carburetor cleaner. Even the fuel line could be blocked. Clean the air cleaner. Also use new gas to operate lawn mower as the current one you are using might be contaminated.
If you're engine is surging, like waivering while running, it sounds like a gas to air combination issue. You may want to check your air filter and make sure that the mower is mixing the gas with the air properly for ignition. You can also run the choke longer.