1994 Ford F-150 oil pressure

This vehicle has a oil pressure problem. It starts and runs great. Has good power and everything. I just recently purchased it and noticed after driving awhile that the gauge was hovering around the low mark regardless how my rpm were. I replaced the assending unit and it worked better. WEll then tonight I was drving it and when the engine became normal temp the gauge would drop down at idle. Engine wouldnt stall or even run bad. No knocks that I know of. I rev it and the gauge goes back up. I have read bearing problems, bad shaft in pump or I am fine and need a new gauge. Any thoughts? The truck is spotless so it doesnt seem mistreated.