Replacing my oil pressure switch today. lol

i just bought a 1999 ranger 3.0 2wd. had syncro issue fixed that. now when i drive about a half hour the oil gauge jumps like crazy. changed the sensor in the back of engine. it lasted longer but then started acting up again. now when i change the sending unit to a proper one and it shows the pressure dropping what should be my next step? i plan on trying to do a flush on the engine with seafoam then putting some 10w30. maybe some lucos oil. need help dont want to change engine unless this one cant be saved. and changing oil pump dont want that unless thats deff the problem.