Paul
That oil pressure symptom sounds like a failing sender,poke around ,i think its around the oil filter housing.When you find it ground the wire that connects to it,that should get you a full scale reading on the gauge as soon as you turn the key on. Catch it at a time that it is not showing oil pressure and ground it,if it then goes full scale it is either a bad sender or an actual (unlikely)oil pressure problem. With a real oil pressure problem you would have a very noisy engine,bad enough to scare you! You could temporarily install a gauge in the hole where the sender goes and verify the actual pressure. Don

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97 E300D 157000 miles
87 300TD ?141k? miles