It's most likely a mag problem, your governor gets its "RPM signal" from a second set of points in the R/H mag (if I remember correctly). When you have a governor problem, the mag is the first thing to check and 90% of the time, the mag is the culprit. I have had, on a couple of occasions, the governor control box go out on me (which is rare but does happen).It resulted in a gradual increase in rpm and was easily detected by the pilot. Its located behind the L/H rear seat back and is (except for one screw) quite easily replaced.
I'm curious to see what they find, so keep us posted.