While the engine was in for tune-up, Did the repairguy change the gas tank cap or put other gasket in it? I have had the same trouble you are describing and it was nothing more than the gas tank developing a slight vacuum. I know that sounds to simple but try to run the unit with the gas cap loose on the tank and see if it still does the same thing. Just be careful not to loose the cap during testing...Don't ask me how I know.
Regards,
Everett R
Regards,
Everett R