1) Does the car turnover fine when it isn't starting (sounds normal when turning, but not starting at all)
2) When it does start, does it sometimes cut out again after a few seconds?
3) Will it stay running only when you give it plenty of gas and RPMs?

If 3 is correct, then it might be your AMM (if there is one in a 740)

If 1 and/or 2 is correct, then you might want to check your connection at the ECU. In my 240, it is a black box just below the windshield wiper fluid resevoir. There will be a plug with a bunch of colored wires going into the ECU. Next time your car doesn't start, go and wiggle this plug and then try ot start the car. Try this several times.

After having my car towed several times and endless mechanics not finding the problem (of course, my car would magically start once off the tow truck), I finally found the problem at the ECU connection. However, I haven't found a fix for it (see my post) yet. But at least I can get the car going again when it does fail.

Because the car started on a shot of ether this is a long shot but it won't cost you anything...

The radio suppression relay on your car is mounted on the shock tower on the passengers's side. There are two identical black box relays about 1 x 1 x 3. Swap those two for each other and see if the car starts. If it does you can probably solve the problem by soldering both relays as John has suggested.

Randy

It will run with a bad Engine Coolant Temperature Sensor, but not well. Catastrophic failure of this sensor is unlikely. I would check the RPM sensor. The early ones were marked with yellow, and the updated ones with white. It could also be the Power Stage (ignition amplifier). Have you had any of the wiring harness connectors disconnected form the engine?

What error codes are you getting?
Look in the 700/900 FAQ's for instructions on retrieving and reading error codes