I've got a puzzling problem with my '86 240. Once it warms up it loses oil pressure quickly and completely. Here's the rundown:

I hadn't driven the car for several weeks. It sat out near the road, pointing slightly upwards for about three weeks. One morning I decided to start it just to make sure the battery was good. The instant the engine caught it mad a moderately loud, very metallic racket that lasted maybe half a second. Before I could react, it quit, and the engine started and ran normally. I decided that something had drained out dry and made a racket before it got a supply of oil. I looked under the hood for broken bits of mangled cat or other foreign bodies, but found nothing.
The next day I drove it to work, a total of 3 miles. Shortly into my drive home that afternoon, the oil light came on. I pulled over and checked the level. As I expected, it was full. I decided that the problem was probably in the sender circuit, and drove the remaining 2 miles home. That weekend I trouble-shot the sender circuit, and couldn't find a problem. With the sender out, I cranked the engine for a few seconds, and nothing came out the hole! The end of the sender was dry, and kind of baked looking. I rolled the car into my shop and began to psych myself up for exploratory surgery in the crankcase.
I expected to find something broken about the oil pump, but couldn't. I had it completely apart to inspect it. It showed no alarming wear. I could still see machine marks on any critical surface. The overload spring was strong, and the over-pressure plunger looked fine. I pulled out the drive gear and it's shaft, to see if the tang were broken off. Nope. I could still see gear teeth on the intermediate shaft. Clearly the intermediate shaft is turning, or the distributor wouldn't work. It ocurred to me that if the intermediate shaft was broken across the center journal, it would explain some aspects of the mystery, but that's a pretty far-fetched idea.
The engine was VERY grungy inside. Up aginst the inlet screen of the oil pump was packed a bunch of stuff that looked like coffee grounds. Pump starvation due to cloggend intake? Eventually I found a hole worn in the intake tube between the AMM and the throttle body. This explained an intermittent cold stumble that had plauged this car off and on for two years, and the presense of all this grunge, so I congratulated myself on a successful surgery, cleaned everything back up, replaced the holed hose and all the seals and gaskets I had violated, and put it back together.
After warming up in the driveway for five minutes or so, the oil pressure light stayed off. I took off for a victory lap around the block. About halfway around the block, the oil light began to flicker, and then stayed on! As luck would have it (I'm needing all I can get), the first half of this lap-around-the-block is uphill, and so I shut it down an coasted home.
I berated myself for all the work I put into this car, taking the oil pump out, etc., only to find out that I didn't solve the problem. Sure, I solved a serious problem along the way (the leaking intake hose, and the accumulation of grunge in the crankcase) but the problem was still there!!!!!
And now I was full circle back to the theory that there is an intermittent short in the idiot light circuit. In the meantime I had acquired an oil pressure gauge and new sender to confirm that theory, so I set about to hook it up. In the back of my mind I remembered that after removing the sender and cranking the engine, nothing came out the hole. That didn't fit the bad-wire theory, but I decided to worry about that later since, after all coincidences do happen, right? Grunge AND a bad wire? And what about that awful noise I heard when I started it up after three weeks of sitting? Lots of loose clues are still lying around.
After cobbling through some mounting issues, I had a gauge taped to the dash, and upon starting the engine, it showed a healthy four bars of oil pressure. The sender I bought has an idiot light terminal as well, so I hooked it up too. After another five minute warm-up, the gauge stayed steady at 3-4 bar. The idiot light was cold and dark. It was time for another ride around the block...
Within a hundred meters of the same spot where the light came on in the previous test-lap, the gauge started dropping fast. When, after five seconds or so, it got to just under a bar, the idiot light came on (.7 bar is the spec). Now there was no doubt in my mind that the oil pressure was indeed gone. The guage isn't lying. I shut it off and coasted home.

Later I found that I can replicate the problem any time, sitting in the driveway, as follows:

1. Start car. Oil pressure springs to 4 bar.
2. Let engine idle. Within five minutes or so, the engine is nearly up to normal operating temperatuere, and the pressure has dropped to around 3 bar.
3. Rev engine to 2000 rpm or so, cycling up and down - within a couple of minutes, the oil pressure will very suddenly fall to the warning level.
4. Shut engine down
5. wait about a minute
6. Start engine up again - pressure returns to 3 bar. Repeat steps 3 & 4 as many times as you have patience for.

Another observation: After step four, the engine doesn't want to start immediately. It cranks for a while, sputters, catches faintly, and gets going after a little encouragement with the throttle pedal.
I checked the plugs, they were kind of blackish, perhaps from all the starting and idling? I also checked the compression - it looked good and even on all four cylinders.
Broken down to the essentials, it seems that either the pump just all of a sudden quits turning, it loses it's oil supply, or a big hole opens in the block somewhere upstream of the filter (which I've changed twice, by the way).

Any ideas out there?
Thanks in advance for any theories. I'm fresh out, and hate to pull the engine apart again until I have a solid suspect in mind.