Dan, I am having the same exact problem with my '86 740. The car always starts and idles fine but after about 5 minutes on the expressway it starts missing, not all the time but it's getting to be more and more of the time.

I reasoned that it was an ignition problem because the tack jumps around independent of the engine speed and the tach is electronic, driven off the ignition system. I replaced the distributor with a junkyard one and the problem remained. Still thinking it had to be the hall sensor, I replaced the hall sensor and the problem still persists. I have replaced the ignition module twice, the last time about a year ago. Both times the module failed the car would not start at all.

I have replaced the coil, plugs, wires, cap & rotor and hall sensor so far. I am beginning to wonder about what role the knock sensor plays with the ignition and if it might be sending a false signal to the ICU. I'm planning to swap out the ICU and knock sensor next and after that I can't imagine what is left.

Don't you just love intermittent problems?