Snow Blowers

I have an 1827sb that has worked great since I bought it. This last month it has started acting unusual. It will start fine and run well for a while then the engine will start revving up and down. It will do this for 5- 10 minutes and then shut off completely. It will restart right away and run rough until it shuts down again. The loger I leave it before restarting, the longer it will run before shutting itself off.
I have changed the oil and the spark plug, and I have also loosed the gas cap when it starts as I thought the cap vent may be clogged, but the issue persists. Any ideas?