Well what the hell is it !…… you say that you have replaced all of the bearings
And looked at all the journals and they seem to be ok, well what ells can it be?
I suppose it could be low oil pressure,(12 psi I have seen lower) but we are clutching at straws now. What pressure do you get at 3500 rpm?
If it was that low the engine would of expired by now?
I think if the big end bolts/caps had become that lose the whole thing would of gone bang “end of story! …..”
You say that you have changed the head so that rules that out.
When you had the head off what did the boars look like?
Were there any scoring or bruising marks on the boars?
Other than that change the block!!….. But that is almost admitting defeat.

Let me know what you find out,good luck

CheeRS

Steve

P,S spec to follow