oil pressure problems

Im trying to help out a friend of mine with a 94 crown vic. He heard the lifters rattling, thought he lost oil pressure , pulled over & shut it off. Had it towed in & the Mech. flushed the pan out, changed oil unt filter & the problem was gone until about 10 miles down the road it did it again. Long story short is, one time it has pressure when started & then next time it doesnt. Any help would be great!

Was the oil pump changed? What weight oil is he using? What brand oil filters? Has he actually measured oil pressure? Did he use any kind of engine flush? Are the oil galleys plugged or restricted?