You may have too much oil in the pan. It may be getting churned up by the crank at higher rpms greating a lot of air bubbles in the oil in the pan which will reduce the oil pressure.
You might want to look at the oil on the dipstick after running at a low rpm and check for bubbles. Then do it again after running at 3500rpm and see if there is a difference? Might try dropping the oil level by a quart temporarily and see if it makes a difference.
Just a thought FWIW