my 400ci did the exact thing 5 or so years ago..it was a melling pump also...i replaced it with a moroso and never had any more problems but i have seen mellings run with no problems ( anything can malfunction)..i guess this is the reason for oil gauges...lol
with the moroso i purchased the pick up tube had a bracket welded off the pick up tube to the pump housing and had a 1/4 bolt that held the pick up tube in the pump...there is pic of this pump i 'm speaking of in JEGS book ..page 56
i did assume you were speaking of the pressure dropping as the rpm's increased but had to ask anyhow