[PontGuy]

I've been watching along quietly in the background to see where this goes. Here is a pic of what can happen if a piece of hard metal trash gets sucked into the pump. No, the grooves in the rotor and bottom plate are not supposed to be there. My engine two years ago. Same sort of sudden drop in oil pressure as described here. It ran great for 1000+ miles before things went wrong. Damage was caused by a tiny piece of metal shot.

Sounds to me like this is a similar story. If you have exhausted all the simple possibilities then suggest that you pull the engine (glad I did!) Start by disassembling the pump and work your way through the bearings and see what you find. In my case I found more pieces of the metal shot in the oil pan which were obviously left in by the builder, so the engine was completely torn down and cleaned. No additional damage was found during the teardown. A new pump was installed and the oil pressure has been back to normal ever since.