Crank shaft plug loose
I wish I would have known about your problem before you went through all of this. I had a exact same problem of intermittent loss of control of the Prop which would go to fine pitch but would only do it periodically. Not ever flight and never could be repeated. The engine (Superior IO 360) had been overhauled and the crankshaft was just polished in the rebuild. The existing crankshaft plug was not removed for this. I had experienced loss of control of the pitch once before the overhaul. I flew the plane for about a year after the overhaul and when ever it would happen I was always able to recover control of the pitch by throttle bursts etc. Occasionally when cycling the prop before takeoff, it would not respond during run up immediately but would always work the second or third time I cycled the prop lever. Finally, on a climb out of Pubelo CO it started to runaway and I was unable to regain control. To make a long story short, prior to this, I had the governor overhauled, replaced the prop with a new MT 3 blade and was still experiencing the intermittent problem until this day when it would not respond at all. Flew home on limited RPM and decided it was time to figure it out. I fabricated the plate on the back of the governor and ran the pressure check and it was ZERO and I could hear the air leaking into the sump. Pulled the prop and sure enough the crank shaft plug was rotated half open. Ordered a new plug, fabricated the installation tool and pounded the new plug in and it has worked fine ever since. What was happening was the plug was rotating like a butter fly valve in the crank. Some times it would rotate closed and oil pressure would hold it tight enough for oil pressure to build up. But when the engine RPM would vary or torqued while in formation etc, it would occasionally open and I would lose oil pressure to the prop. By changing the RPM it would rotate closed again and I would regain oil pressure. What was so aggravating was it was intermittent. Just thought I would share my experience.