88' Sable turns over when it wants to

I have *THE* weirdest issue with my car, the mechanics can't even figure it out!!! It only turns over when it wants to, or so it seems. I have had the alternator replaced, battery checked I think by now 4-5 times...I lost count...it is holding a charge just fine and there are no draws off the battery. I had the terminals replaced, and the battery cables checked and ground connections cleaned. I have had the starter checked, I replaced the alternator within the last week and a half...the car will run fine for days, and then it just won't turn over, all of the sudden. I had to have it towed twice in one week, the first time was before the alternator was fixed. I drove it for four days after and it was just fine. I was driving the other day, parked on the street for 5 minutes and when I went to start it, it wouldn't turn over and then the battery went dead. This is not the first time this has happened. I stopped to get gas a few times and it did the very same thing. ANY ideas??? Any and all help would be much appreciated...I am soooooo lost here... :confused:

My first thought would be the ignition switch. Not makeing a good connection somewhere, switch not turning completly off or grounding out and running the battery down. I don't know how the starting system is setup but it may be possible that you have a bad starter relay. If there is one.