I have a 2007 4.0 v6. Engine was rebuilt and after many timing chain tensioner issues was running fine. Last week engine oil pressure light and gauge report no oil pressure. Oil was just changed two weeks prior, checked again and was full. Replaced oil pump. pan was clean as a whistle. engine started fine, oil pressure came up. no problem. About five minutesg later oil pressure dropped to zero. Ford dealer says issues with oil pump drive shaft can fatigue and then jam. Replaced it. Same results. starts fine, pressure is good then disappears. Going crazy. Any ideas.