I've seen on a few 44's & 22's where there is a miss-match between the two needles during normal flight. You start the flight and both needles are "top of the green" then after a while the ERPM begins to either increase or decrease by as much as 5 - 6%. No change in the RRPM and the engine was running normally. Never really found out what caused these issues, instrument or pickups?
May-be this?