I have a 2008 John Deere 3720. I am having trouble with the tractor shutting down. It started last winter, shut down while I was blowing snow, just like the switch was turned off. I was able to restart the tractor, cranked over about 5-10 seconds and started. Sometimes ran fine, other times ran for a couple of minutes and shut down. I changed the fuel filter, drained the fuel and replaced with new, cleaned the sediment bowl. I even added some fuel/injector cleaner additive. It ran fine until last week. Repeated the same problem only this time shut down every 10-15 seconds. I nursed it back to the garage. I checked the safety switch on the seat, looked fine. Removed the fuel cap, blew out thinking the vent was plugged and creating a vacuum in the tank. I started the tractor yesterday, cranked over more than usual to start but ran fine for an hour. I am waiting for the next time it quits. What am I missing? Any suggestions will be appreciated.