Scott, I am having a similar problem with an 86 300E. I am thinking I have an ignition problem because when I got the car back home I gave it a short shot of ether and still had the same symptoms. After it quits, it seems to be firing almost randomly when trying to start as it kicks back, belches, and tries to fire occasionally. It will always crank up and run perfectly for about 5 or 10 minutes and then dies. I haven't checked the ignition module for extreme heat as mentioned but am thinking the ignition module or the crank speed sensor may be acting up. My next step is to warm it up and put a timing light on and see if I can tell what is happening to the timing. If I have to replace the ignition module, based on the price of the bosch unit or replacements, I think I will try to go to a Crane or other after market unit. Then I hope to find someone that has been there before. Looking at the Crane XR700 as a possibility.

Good luck and let us know what you find.

John