I have a problem with my 1991 525i E34 M50 that is puzzling, at least to me. I have searched the Internet and this forum for a similar situation, but could not find one.

Last night as I was driving home from work, the SRS light on the dash came on. About a kilometer later, one of the other dash warning lights came on and then then I noticed that the headlights were getting dim.

As I braked for a traffic light, the engine started running very rough and the whole car started shaking as the engine surged. When I turned off the lights and heater, the surging stopped, but started again when I turned the lights back on.

When the traffic light turned green, it took me about 20 seconds to crawl through the intersection. Fortunately, there was no traffic and I limped to a place to stop. By the time I coasted to a spot, the engine had died and there was very little electrical power left to illuminate the interior lights.

My first thought was the serpentine belt had broken and I had been running on battery power alone. When I opened the hood, everything appeared as it should. The belt was intact, there were no signs of any fluids leaking, and the alternator was cold to the touch.

After about five minutes of scratching my head, prodding relays, and tugging on cables, I noticed that the interior lights were now bright. I turned the ignition key and the car started fine and ran as if nothing were wrong. Even turning on the headlights and heater did not cause any changes in RPM or cause any warning lamps to illuminate. Since I lived about one kilometer away, I drove home without a problem.

Today, I decided to drive to the next town, which is about 15 kilometers away. As I neared an intersection, the SRS light came on and about 30 seconds later, all of the dash instruments died, followed quickly by the engine. When I turned the key to the run position, the lights and gauges on the dash were dead. When I tried to start the engine, there was nothing.

After about ten minutes, it appeared as if all power was restored, except the car still would not start. When I turned the key to the run position, all of the normal dash lights came on and the gauges were active. When I tried to start the engine, the starter motor would engage, but after one attempt to turn, it would stop. It had the appearance of a dead battery.

I called ADAC and waited for about 15 minutes, all the while making new friends at the busy intersection. On a whim, I tried to start the car and as it did last night, it started without a problem.

About ten minutes later, when I was almost at my destination, the SRS light came on, but I had parked before the total power loss occurred. The car remains where I parked it and my wife picked me up.

If I can rule out demonic possession, what could be causing this problem?

If it helps, here are more particulars about the car.

It has about 295,000 kilometers on the original engine
Automatic transmission (replaced ten years ago)
New thermostat (previous thermostat failed open)
I keep getting the coolant level warning, even though the level is fine.

I appreciate any help!