Big...you said the plate on the inside is fine, and you have inspected it...so that means nothing is between the plate and the casing...and you inspected the casing for cracks....I am sure you have already heard or tried this, but I am just going over it again as it baffles myself as well....when you checked the pickup screen, could there be a crack in one of the two rubber sides that clamp around the pump( if I am thinking of the right oil pump) SG made mention of calling a dealer...I looked for a service bulletin and didnt see one...but not to say there isnt one.....
If you can...try another bar and chain of a buddies just for the heck of it....in Oregon(and I think someone said this) it needs to be a K095 mount pattern...
good Luck