Ok, you may have tried this already and yes, I see you replaced it already once, but.....(and I KNOW you checked the injectors to see if they were firing, and you checked for spark, but try this anyway) replace the module for the distributor with a known good one. I had a similar incident before, and thats what it was. I had spark at the plugs, but it wasn't when I needed it, and I had replaced the module with a new one a few weeks prior. Replacing it fixed the problem.