My 1992 BMW E34 525i with M50 non vanos engine will not start. The symptoms: The check engine light does not illuminate at all preventing me from doing a stomp test. The engine does turn over, but does NOT fire or start.

The last time I drove it, some lights on the speedo were flickering. The ABS and check engine light flickered. The engine actually cut out momentarily
on the highway, but started up again without having to pull over.

Just to make things more complicated, here are some things I did to the car recently:
1. replaced fuel pump and fuel level sensor. This seemed to impove engine performance. The engine had been stuttering while pulling away from stop lights. The fuel pressure must have been low. The gas gauge had been flaky. The new fuel level sender seems to have improved this behavior, but I have not been able to run the gas low in the tank in order to test this since the car will not start anymore.
2. I wired an electic inverter directly to the battery under the back seat so I can charge my electric golf push cart battery on the way home from the golf course. It has automatic shut off in case the battery gets low. Plus, I have one of those voltage regulators bolted on there. I hope I did not damage any of my electronics with this thing.
3. I switched out the master and slave brake cylinders recently and got some air in the ABS unit along the way. I have not had time to take it in to get this resolved yet. I hope the ABS is not sending some sort of kill signal to the ECU.

These 3 things probably have nothing to do with anything. I suspect that my ECU/DME/Motronic is bad, but I am not sure. My car is so flaky, I usually think the engine harness needs replacing when something quirky starts going on, like the speedo lights start flashing. I guess this can be expected from time to time since my car is a 1992 with 250K miles and is parked outside in the elements.

Has anyone had similar symptoms? If so, do you how I can narrow down the problem?