I had almost exactly the same symptoms: fluctuating oil pressure and very rough idle when warmed up. This followed head work and gasket replacement after an overheating episode.
My mechanic suspected the O2 sensors, but it turned out to be the idle speed regulator. Specifically, the insulation on some of the wires in the connecting harness had become brittle and was allowing sporadic contact. My mechanic discovered this by noting that the idle was affected whenever the harness was shaken. Opening the covering of the harness revealed the shorting wires!Really sharp work by my mechanic.
The solenoid for the EGR valve was also replaced.
We found that the oil pressure problem disappeared when the oil was changed and 20W50 added.
Hope this is helpful. Best of luck.