Sounds as if you have some little gremlins in your electrical system. If you haven't already, check for fault codes in the computer. If you find any, write them down and clear the system to see if anything recurres. I'd also clean all the ground connections you can find in the engine compartment. On our 91 740, we had a random stall problem that turned out to be the FI relay which you have already resoldered. We also has a problem related to the connection at the AMM which revealed itself with a fault code. I cleaned and shot some contact cleaner in the connection and she fired right up and it hasn't recurred. After it stalls, you can pull off this connection and see if it will start which converts the engine into limp home mode. You can also try switching the leads on the RS relay and fan relay and see if it will start. Have also read that the connection to the RPM sensor can become contaminated with oil, corrosion or whatever. Doubt this tells you much you don't already know