Lawn mower not running well!?

I have a Briggs and stratton lawn mower that is just a year old. the mower started hesitating a lot, going to low rpms regularly then jumping to high rpms then will shut off in thick gr that it would have cut easily in the past. It is burning gas very fast about twice as fast as it did before I have changed the oil, spark plug, and cleaned the air filter and carburetor. Any thoughts on what the problem may be? Should i just say F%$^ it and buy a new mower?