2001 Jeep 4.0 Oil Pressure

This is a new one on me. About a year ago while driving the oil pressure switch just sunk down to almost nothing. Shut off the engine checked the oil; full. Started it back up, oil pressure great. Ran that way for, let's say 3 or 4 months. Then the pressure fell back to zero. Checked the sending unit, OK. Started it back up, 40+ PSI. Ran it for a while longer. This went on for a while (once a week or so), always while the engine ran great, no noise. Then, while driving, oil pressure drops to zero sunddenly and the check engine light comes on. Shut it off, let it cool, lifters rattled bad after re-starting. Let it sit, next day started it, no noise, engine ran and oil pressure dropped to 7 PSI on a gauge directly connected up. Shut it off, 3 days later, drove it ten miles home, (thermostat was open; engine at 190 or so), oil pressure stayed right at 40 PSI and it ran great, let it run a while longer, probably all together, 30 minutes no problem. ( I must say, when it dropped the last time it took only 5 or 10 minutes to fall. Intermitted problem somewhere, 100,000 miles on it (not much for a jeep 4.0) and oil changed regularly. What's up with this?