Bill,

Sure sounds like you have covered most all the bases with this cut off problem. These are a real bear to track down as you about have to have the car on an analyzer when the problem occurs which is not likely. You're right in that if it has gas, fire and compression, it has to run. The RPM sensor is a good possibility and makes one wonder, as you suggest, if something in'ts heating up and shorting out. I haven't looked at the RPM sensor on mine in years and forget exactly what the hook up looks like but being behind the engine it can certainly have worked loose or the connection contaminated with oil etc. Another possibility is a short in your ignition switch. I've heard of cases where folks had a huge wad of keys on their key ring with the car key and the weight of all that mess swinging back and forth over time shorting out the switch but that'd be a real long shot. I went to the trouble of bypassing (aka hot wiring) my ignition switch to track down this cut off problem in ours that turned out to be the FI relay. All this does is but 12 volts on the coil pac without going through the ignition switch.

To check the computer fault codes, (I assume your 90 is the same as my 91 740) locate the diagnostic box which is a small black box mounted between the drivers side strut tower and the firewall right below the hood hinge. Remove the cover and you'll see a number of round sockets with number beside each, a LED and button along with a small "pigtail" with a wire attached that is stored in a holder in the cover. I'm doing this from memory and think this is correct. I'll check in the AM to make sure. Pull the pigtail out and insert it in socket #2. Turn your ignition switch to on but not start. Push the small button on the diagnostic box for at least a second but not more than 3 seconds. Release the button and watch the flashing LED. It will flash a code with 1 to 3 flashes followed by a pause, 1 to 3 flashes followed by a pause and one to three more flashes. You need to count and write down the number of flashes displayed in each segment which will give you something like ( ** ** ***) 2-2-3 or
( ** * * ) 2-1-1 etc. A flash code of 1-1-1 means the computer has no codes stored and it can hold up to three in memory. To clear them, hold the test button down for 5 seconds I think. Then move your little pigtail to socket #6 and repeat the procedure. Once you have your codes, you can look up the fault on various charts that will steer you to the component that needs attention. I wouldn't be surprised if you come up with all 1-1-1's (normal). I had normal 1-1-1 reading and our car was just cutting off at 70 mph on I 95 or where ever it wanted. You would coast down the road, pull off and wait 5 to 30 mins and it would start right up as if nothing had happened. That was the bad FI relay but it triggered no codes. Don't know if any of this will help or not and if I'm wrong here, someone please correct me.

bruce