Why won't my car start?

[JMASE6]

I have no idea why my car won't start...

One morning a week or so ago I put my key in and got a slow crank then a start. Then it was a slow crank and no start. Then I got it jumped, turned it off and like magic it started fine. So I drove to the auto store nearby and they said my battery tested fine and since I had an ugly terminal connection on my negative battery cable to replace it. After fixing that, it drove fine for the rest of the day. The next morning it wouldn't start. So I got a starter from a salvage yard (assuming it's good...). No start, just a click. Replaced the positive cable because I had a feeling it might be bad and that the poor connection might be why it wasn't starting. Nothing. Not even a click. But it did jump. And after removing the jumper cables it died less than 30 seconds later. So I replaced the alternator (also from the salvage yard). Still, not a damn thing.

I just do not get it. Any suggestions? Anyone? Please help. My friends are getting tired of me asking for rides. Haha

Thanks