Oil pump, I've never seen one go up and down, just low pressure constantly. All it is is two gears that mesh and turn to move oil. They do wear out, and should be replaced when the engine is rebuilt.
Low pressure due to bearings is usually due to worn cam or main bearings. I'm trying to remember for certain the oil flow on a 235, I think the oil passage for the #6 rod comes through the rear main bearing. If that bearing is to loose, the #6 rod would not be getting enough oil. If I'm wrong on this, hopefully someone will correct me. My books are at home, I'm not.
On your mechanic. Have you done business with him before? What is his shop like, neat and clean or are there parts piled high and a floor so dirty you can't see it? These are things to look for when picking a shop.
