fatal electrical(?) mystery
 

I have a '92 Miata sitting immobile under a tarp in my backyard. The car failed while driving down a mountain highway many miles from home. I first felt a judder, then looked at the tachometer and saw the needle moving spasmodically. I accelerated, and it seemed to regain power for a minute or two, then died. When I turned the key, it immmediately restarted, but died again after a second or two. And that's the way it behaves to this day, whether I crank it repeatedly, or let it sit for a month or two. It immediattely fires up then quickly dies, usually after 2-5 seconds. The longest it's run was perhaps 15 seconds. When it is running, pressing the accelerator has no affect, nor does spraying ether into the intake.

I've taken it to two mechanics. The first replaced the igntion module and CPU with units borrowed from a running Miata. Neither made a difference. I had it towed home and, in desperation, replaced both units with eBay-purchased parts from reputable sellers. I also replaced the cam angle sensor. Nothing made a difference. I later towed it to a second mechanic. He was absolutely convinced it was the cam angle sensor. I told him about having replaced it and suggested switching the CAS with one from my '93 Miata. He did that, and it made no difference at all. After that, he was out of ideas.

This is what the second mechanic tells me: The compression is good. Fuel pressure is good. Spark is good momentarily, but then cuts out. It seems to be in otherwise good shape.

I need to emphasize I am not mechanically inclined, and my familiarity with diagnosing electrical faults is ZERO. I'd like to get this running, without spending more on it than it's worth.