IMHO, you should absolutely do this as an experiment to test the theory of the pickup being too high. With the stock pickup being up .3" off the pan, you lose about 1/2 quart of oil that can be picked up.
If this erratic pressure issue goes away, then you have answered the question. If the issue stays the same, and you don't have any missing plugs anywhere, then it is likely the clearance issue.
If I may ask the question once again, when you exit the freeway and come to a stop and the pressure is low, does it stay low for as long as you stay stopped and idling? Or does the pressure just drop low for a few seconds and then come back up while you are stopped and idling? That answer may tell us more about the specific issue.
