I have an approx 10 year old Murray lawn tractor with a 20hp Briggs & Stratton twin cylinder engine. It has been running uneven for a little while. It will rev up and down in about a 2 second cycle. If I have a load on the engine (i.e. in drive with the blades engaged) it will run just fine for about 20 minutes. After that it basically just stops. I have put in new gas, added carburetor cleaner to the current tank, cleaned the plugs (they are new this spring), checked the fuel filter, and cleaned the air filter (new this spring). Once it dies, I can restart it, but it will only run for a few seconds. After a day or so I can get the same 20 minutes out of it before it dies. Any tips or suggestions?

If the cleaner didn't help ,you need to clean out the carb,sounds like the jet isn't all the way clear ,its notting getting enough fuel.