This morning (Saturday) the car didn't start at all. The starter cranks but engine won't turn over. A couple of times it seemed like it was going to start, nope not today. The fuel pump test light was lit, fuel tank ~1/4 full. Now that I think about it there have been a few times that it had some difficulty starting, but eventually caught and ran OK afterwards.

Last summer (in an attempt to solve this same ongoing issue) my mechanic tried swapping in an ECU from a FLAPS, but he said it ran better with the original one which I believe is from the '91 LSC (this was a Converse in-house conversion). Maybe the replacement was defective off the shelf?

I still believe that it's something (relatively) simple, what am I missing?

Possibly tune-up related (plugs/wires/cap/coil)? Is there a throttle position sensor on this engine? Ignition module?

The symptoms are "all or none", meaning that it's either fine or or it's running like crap (not a gradual degradation).

Argh.