2000 Pontiac Grand Am stops running

Hi, the problem with my gran am is, that it will run fine for awhile, then without warning, it just shuts down. No engine light comes on, no gauge lights come on. Up hill, on level, etc, does not matter. After a few minutes it will start, and be fine again. Now the first time it happened, the gas gauge read 3/4 of a tank. Filed it up, ran great. Happened again, fuel gauge read the same amount of gas. Filled it up ran great, and it is only taking around 3-4 gal, so gauge is ok. Now, took car out this morning to go to church, gas in it now is almost full, well, it shut down again. Fired up, and have not drove it since. Does not matter if it is a hot engine or cold. What I did a few weeks ago, is added some peak coolant to it. It said good in all cars, because I know it uses dextrol, and did not add much, but could this be affecting the coolant sensor, and screwing with the fuel displacement. Or could it be possible I an drawing water threw the lines now and then. See, I never would let the gas go below 3/4 cause it is cheaper to fill up, but, the day I drove it on the turnpike, I was just going to fill it up when I got back, but this is the first time it shut down, running about 75mph. It just stopped, coasted of to the side, and after a few try"s, it started, and ran great. Then I was getting back on the pike, and it shut down again. Fired up, got it to a bp station, filled it, and got me home, about 35 miles, and no problem again, until last week, same marking on the fuel gauge, filled it up, no problems, till like I said earlier, it shut down this morning, and it is almost full.I am scratching my head with this, and the car is in very good shape, and I take very good care of it, so what do you think? Sorry for being so long. Thanks, don