low oil pressure

Try revving the engine in neutral. The oil pressure should increase with RPM up to a point and then hold steady once the bypass opens. If you drop pressure as revs increase, especially above 3500 RPM or so, this is a classic symptom of excessive rod bearing clearance. Another quick check... add a quart above the level you run now. If this fixes the problem, you are uncovering the pickup. Don't run the engine hard if this your problem! You will starve it for oil when it needs it the most, dramatically shortening the life of your bottom end.

BTW what is your peak and cruise oil pressure reading? 15 PSI at idle (warm) is plenty.