[mshac]

Sorry to hear of your troubles, BILLS.

Since the prop hub is hydraulic, and requires oil pressure to move the blades to a lower AOA (higher rpm), we can assume the prop hub is not getting the required oil pressure, for some reason, after it warms up.
If the prop hub fails to receive enough oil pressure, the blades will rotate to a higher AOA (lower rpm) until they hit the feathering pins, or "stops".

Since your prop blades go to un-commanded full feather, it would seem to me the pins are not rigged correctly. There is no "auto-feather" on these props! The pins shouldn't release until the prop control lever is in the feathering gate position. We don't want our prop blades going to feather without pilot input, because what if we want to air start? Lets say a tank ran dry, so you switch tanks, and now the engine has fuel again. With the prop windmilling, it will start right back up once it has fuel pressure. Feathered, we'd be hoping the starter and battery were up to the task of turning the motor fast enough to start while the blades are in feather.

IIRC, the props must be turning at 600 rpm or greater in order for the feathering pins to release. This keeps the props from feathering when the engine isn't running.

When running the engine, does the prop slowly move to low rpm over a period of time, or does it release all at once? How does the engine's oil pressure reading change over the 10 minutes? After the prop feathers, while the engine is still hot, are you able to manually move the blades out of feather with a prop paddle or such? Do they stay out of feather (ie do the pins function)? These could be a clue...

Black oil normally indicates excessive combustion ring blow-by. The carbon turns the oil black very quickly. Even one bad cylinder can turn the oil black in a matter of hours.

The sludge is what would concern me. That can clog oil passages.

I would drain the oil through a sieve, collect anything you find, and send some oil in for analysis. If you've got a dry/spun bearing, it will show up in the report.

You may consider adding a quart or two of avgas to your oil and warm up the motor before you drain it. Should help it drain more thoroughly.

I would also run a quick compression test on all the cylinders. You didn't mention if the motor had been broken in yet? Hopefully this wasn't just a "spray paint" overhaul!