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?