Engine dying, electrical system fine

[Torchiest]

My first post, and apologies for any newbie mistakes. I'll try to put this in the best chronological order.

2-3 months ago, 2005 Focus wouldn't start. My brother and I jumped it, and we went to Autozone and had them check the battery. It was fine. The guy claimed the alternator was bad. The next week, we took the alternator out and took it back to get a replacement, but when they tested inside in the store, it was fine. The car seemed okay after that, and I shrugged it off as maybe leaving an accessory on overnight. It may or may not be related to the current problem.

My car has died on me twice while driving home from work in the last few weeks. Both times, all of the electrical systems were still running, but it would not restart. No clicking, nothing. After a few minutes, it would start back up. The second time, I was on the freeway, and I saw the battery like come on for a second, then a few minutes later come on for like five seconds, and then a few minutes later the engine died.

The battery terminals are clean, and a bad starter wouldn't kill the engine while running. Like I said, both the alternator and battery tested fine, and I don't see how they could kill the engine while the electrical systems were still running okay. We had read this could be a problem with a particular inline fuse, but we checked that and it was fine too.

A couple days later, and the car won't start at all, but like I said, the electrical stuff runs fine. The next culprit we were going to look into was a crank position/engine speed sensor going bad. Has anyone ever heard of anything similar and know of a possible cause? Thanks for all replies.