Lawn tractor running poorly?

We had a 10 y/o Toro mower that started to run like crap similar to what you said. We down loaded the parts list from Briggs based on the engines serial number and used those numbers to buy the correct carb rebuild kit. It worked like a charm. Those seals inside start to loose their cohesion and small vacuum leaks start. That throws off the fuel air mix. Just something for you to consider.