Q: Intermittent Low Oil Pressure

I have a 2002 Expedition 4x4 5.4L with about 355,000 miles. A couple of months ago, it would run fine until you would get to an idle while in drive such as at a red light or in a parking lot it would start to shake and almost stall, but besides that it would be fine. Well last week we were on our way to Atlanta for a Drs appointment on the interstate and the oil light came on and it started shaking so we pulled of the exit to a Meinekee where they checked it out and changed the oil. They said that it was black and gunked up. ​​​​​​​We had it towed home, and several people who came to see what they thought it was, said that it was the oil pump. When you crank it up, it runs fine for a second or so and then oil pressure drops and starts running rough. It will crank up fine and sometimes it starts out rough idling and then goes to normal or sometimes it will start up and idle fine and then start lightly knocking after 4 or 5 minutes. Thanks Joshua

My car has 355000 miles.
My car has an automatic transmission.