I have a Toro mower with the Honda GCV160LAO S3B NH1 engine. Recently I've begun having problems with it running and staying running. It will start, the engine then begins surging non-stop, then the engine stops after a few minutes. I have replaced the thermowax pin, cleaned the carb (jet and emulsion tube), and replaced the sparkplug and air filter. Any suggestions would be GREATLY appreciated. Thank you.