318 magnum,is my oil pump failing?

well, I'd say there would be a "check-off" list.

• Verify the gage/sending unit is working correctly
• I would question the pressure relief spring on the oil pump
• Perhaps something floating (???) that now and then plugs up the screen on the pickup tube...??..
  
• I would doubt the oil pump, because they are usually "good" or "bad"
• I doubt the pressure drop (if it's real) would be tied to the intake gasket, but stranger things have happened