misfire

Hello, I have a problem with a '94 VN750 that I have had just a year. At the end of last season it started exhibiting this problem, and it continues this year. I am relatively new to bikes so I could really use some advice in figuring out where to go with this: The bike will start just fine...

2003 VN750 In first or second gear at low rpm, or when backing off the throttle I experience what feels like and sounds like engine misfire. There is no loss of power or stalling. Can't tell if it happens at higher speeds/rpm, too noisy. My mechanic replaced the spark plugs and caps and vacuum...