Did you recently change the timing belt? It sounds like when my cam sprocket dowel pin sheared off, I haden't used any loctite when I put in my sprocket bolt and after a few hundred miles it backed out just enough that the pin sheared, it started great for a while then the ecu kept pulling timing as it warmed and eventually wouldnt start once the pin fully sheared, luckily it didn't damage anything, it just stopped spinning just out of the way of everything, but it backfired like a mother until I saw the gear wasn't spinning and stopped. I was lucky I had an extra exhaust cam