I have a 95 325i Convertible with 91,000 miles. When driving under load the engine RPM cycles up and down about 2000 RPM. With very light throttle applied engine speed is constant and slowly increase with speed. Behaves like this at all speeds, hot or cold engine. This started all of a sudden about a month ago (didn't gradually get worse). No repairs have been attempted yet - any chance there is an easy fix to this?