How low is the oil pressure? Does the oil pressure drop intermittently and does the engine bog at the same time?
My 390 used to
intermittently lose all of it's oil pressure and start running erratic, losing power, at the same time, then it would go back to normal. Long story short, when I rebuilt my engine I found a spacer behind the camshaft timing gear that was causing 1/4" of endplay.

The oil holes in the camshaft bearings were being exposed, causing the pressure drop and the endplay was affecting the distributor timing.
I too would suggest checking the timing with a light and checking the oil pressure with a mechanical gauge.