Alternator, belt or Water in gas problem?

On 9/22/2014 As I was headed to work, I stopped at a local Gas station and filled my 04 Deville up, drove into work with no problems. That night as I was leaving work I sat in my car outside letting it warm up as I normally do, after about 5 minutes of it running it suddenly died, the voltage dropped to 11, 2 volts, I was able to restart it, and took off, I got about 2 miles down the road when it died again showing a voltage drop. ( There were never any warnings of the system not charging or a battery light coming on other than the battery light coming on as the car had already stalled out ) I drove 20 miles home and never had one problem out of it, I was able to stop at traffic lights and it remained at a constant 14.5 volts, never dying. As soon as I got into my driveway and got out to open the garage door, the car died again, showing yet another voltage drop back down to 11, 2 volts.

I got up the next morning and went to work with no problems, no stalling out, no hesitation, no nothing. As I left work last night for my two days off, it ran fine. This morning as I got up to go have a load put on the battery ( which is one of those red top battery's ) it crapped out again, and died before I ever got the car out of the garage. Again showing a voltage drop of 11.2 volts, again no warnings about the system not charging, but this time it was different, there was hesitation and sputtering in the motor as I tried to idle it up. I took it to a local auto parts store and had them load test the battery, it came back to being at 14.6 volts under a load, and 12.7 volts with the car not running. I then had them do a load test for what its worth on the alternator as the car was running, and it came back as the alternator passed.

My question is, could it be the voltage regulator in the alternator going bad, or the alternator as a whole taking a slow crap on me? Or could it just be that there is water in my gas tank that's causing the car to crap out? Or could it be the belt? The belt looks fine, it don't ( to me at least ) seem as tight as it should be, but I'm a big guy and I tend to like everything over tightened.

I don't wanna go buy a new one and tear the car apart only to find out that it was not the problem. I know electrical issues can be tricky to solve, more so just by way of telling someone the symptoms I am experiencing with the car, but I just do not see it as justifiable to tear the car apart yet until I have a general Idea of what the problem really is.

these are all the symptoms I have experienced in the past two days, and it didn't start until I filled my gas tank up.