2000 Plymouth Voyager Doesn't start/shuts off on its own

Engine Performance problem
2000 Plymouth Voyager 6 cyl Automatic 137000 miles

Please forgive my descriptions, I am a computer person, not a car person.
I just purchased this van from a used dealership not even 5 days ago. It seemed to be running fine. Yesterday morning, the engine stopped while I was making a u-turn. I was able to put the van in park and start it up again. That afternoon, however, I noticed a sharp hissing noise, almost like a tea kettle ready to boil over. Thinking it was my engine coolant, I went ahead and filled it up (it was nearly empty) after the van had sat for about 2 hours, just to make sure it was cold. The van started up fine, but in the course of a 2 minute drive from my in-laws to my house (it's not even 3 blocks) the van "shut down" on me 3 times, most noticeably when I was taking a corner. No lights cut off, my AC and radio didn't skip a beat, but the engine treated itself like I had pulled the key from the ignition. Each time I had to put the van in park, give it a minute and attempt to start it up. Each of these three times however, I had to really hold down the ignition a while (maybe close to 15-30 seconds, sometimes I had to stop and try again) to get the van to turn over. I can hear the starter whine and try really harder, it almost sounded like if the car wasn't getting any gas. I have a 3/4 full tank, and I believe my van was suppose to have a fuel injection system? When we (finally) got it home, we got out and noticed the hissing noise was louder than ever. Since the car was in park, the engine ran fine for a few minutes. It almost seems that the clear issue is:
Hissing noise
Very difficult to turn on
Engine shuts down whenever a turn is made (regardless of direction)
Can you advise me what I need to do?