After 17000 miles my bike starting running rough as though the fuel injection was either on or off between 2000-3000 rpm while riding through town. It runs smooth at hiway speeds but any change in throttle is abrupt, again like on or off. When I stopped to fill up the gas tank, I actually had to force open the gas cap due to a vacuum in the tank. The dealer can't find a problem. Now the bike stalls occasionally when coming to a stop, but starts up easily. Still drawing a vacuum in the tank which obviously isn't venting. Any ideas as to what might be causing the problem. I hate to leave for the National Rally with this hanging over my head.