I have a 1991 malibu sunsetter with a mercury 5.7. It is not carb with points. I had id service last year, was runnin great untill it started to bog and shut off at idle speed when I woul go thru a no wake area. It just bogs and shuts off?? when we get it up and running, there is no problem, just when we bring it down to idle speed. What could it be???