Without knowing exactly what has been done to the motor, the first thing that I would be checking is the conrod big end and crank for roundness. And a check for bearing clearance of various stages on the conrod bearing using flexi guage or something simular. As long as the crank oiling galleries are all clear oil feed should not be a problem.

I have seen simular problems before. Once it was because a mate had acid cleaned his block and had not washed it out properly afterwards (caustic ate the bearings pretty quickly) and on another occasion the conrods on a motor were not resized correctly causing the conrod to be out of round on the big end.