I have a 98 dodge dakota, 4 cyl, 2wd. My oil pressure drops to 0 after driving about 1/2 mile and then the engine starts knocking really bad. I had to pull off the highway numerous times, turn off the engine, let it sit about 1 minute and start it back up. The pressure would rise to 40 and after about 1/2 mile or so it would do it again. I had it towed so I wouldn't do any damage to the engine. After I got it home and it was cooled off I started it and the pressure was really high but dropped after about 10 minutes of running. Checking the oil I found that it is showing overfull. Oil Pump?

Sounds like the thing may be sludge up. Can you see inside the valve cover? It sounds like the oil pump is pumping the oil out of the pan and it isn't coming back down- plugged returns the head. You might also have a clogged pickup tube or a bad pressure relief.

When was the last time you changed your oil?

spun bearing, or a stuck oil pump over pressure valve.