What can cause a lawnmower to Sputter?

I have a Torro Zero turn mower I bought used last year. About a week ago half way through the back yard it started sputtering pretty badly then stalled out I had added fuel about a half hour before. I got it started and it puttered back onto the shed. I checked the fins for pluggage and the cleaned the air filters (wich I'd already done at the beginning of the summer), I changed the fuel filter but it didn't look plugged. I'm thinking though it has something to do with the fuel, its fresh but maybe water got in there or there's to much eathonol in the gas. I haven't drained the tank or taken off the fuel lines to check for pluggage yet. Is there anything else I should look at that I haven't? Does it sound likely the sputtering is caused by bad fuel? It sputters and stalls even when the engine is cool. It stalls out faster when the blades are on. Any ideas or insights would be apreciated. I'm going to buy some addative for the fuel tomorrow and change the oil (the level is okay but I haven't changed it in a while).