djxl

My 99 Explorer started this problem recently and it's been getting on my nerves.
This Morning I drove it to work and it got there fine. This afternoon I went back to my car and it cranked but wouldn't start. The explorer would crank and it sounds like it want's to start but won't. I gave it a little gas and it would stay on with my foot on the pedal but would immediately die out once removed. Luckily I live about 5 miles from work so I drove it home with one foot on the excelerator and one foot on the brake.
I had a friend of mine listen to the engine as it went out and there was a small hissing sound coming from the left side of the engine. I thought fuel pump but I don't think I would of gotten home if it was.
My last attempt to figure out what was wrong I began to have a harder time starting it then suddenly it kicked in normally. The car stayed on and I test drove it and everything seems fine. My only problem is that this occurs every at least once a month and it gets more difficult everytime it happens. I can't afford to take it to the dealership because I have a need for my arms and legs. But at the same time I need an idea on where to start working. If anyone has any ideas please let me know, I would really appreciate the help.
Thanks in advance!!