1998 Mercury Sable Alternator, Battery or something else?

Okay so I have an issue with my 1998 Mercury Sable, and having recently been displaced from my job thanks to the economy. I am on a very limited budget, so I hope someone has an idea where to start.
NOW 3 years ago, I had to have the engine rebuilt, since I guess I have aluminum heads and ran the car hot, since then, the car has had some issues, mainly that the check engine light has been on, and it pops that the Cam Sensor is bad.
SO in the last 3 weeks or so, I seem to have intermittent starting problems, it would start and run fine, but sometimes when I would stop somewhere, and shut it off, I come out and it wont start, well it TRIES to start, it will turn over, but not fire, So I get a jump from someone, and it will start up, and off I go, with no problems for another day or two.
SO I took it back to where I got the battery. Dont cry. It was Walmart, they ran a test and the battery checked out okay, so the guy said my negative terminal was loose, so he changed that for me, and after I left, I went about 2 weeks with no problems.
SO then the problem started again, where it would start when IT wanted to, funny thing is is would ALWAYS start at home, just not when I would get to where I was going, and again, not all the time, but off and on.
So yesterday I took it BACK to walmart, they ran the test on the battery again, and again it came back okay, so the installer removed the NEGATIVE battery cable and the car shut off, so he said it was my alternator. Though I always thought it was the Positive cable. But anyway.
After that, we tried to start the car, and it would not start, so they did change the battery to a new one, but it still would not start, finally, the "installer" did something with the fuse box under the hood, he plugged and unplugged what he said was a "relay" and the car started, YAY, I think. I was planning on taking the car to Advance today to have them check the alternator for me, BUT getting in the car this morning, it would not start, it still turns over, but wont start,
i have NO idea why the dang alternator for this car out of all the BETTER cars I have owned is so dang expensive, but would you think that may be the issue or something more serious? Would the cam sensor keep it from starting? Because I was told it would not affect anything except that it would backfire or run rough.
SOMETIMES the check engine light goes off and stays off for weeks, then comes back. So I have no clue. Would LOVE a new car. But need a job first!~ Lol
HELP?