my car is starting then immediately dying.

2 years ago it would crank but not turn over. replaced the ignition switch and it has run perfect until the Saturday. It cranked and wouldn't turn over. Turned off the radio and it started right up. For a couple days it started fine as long as the radio was off. Yesterday it didn't matter if the radio was on or not. I only mention the radio because a couple times, since Saturday, I tried to start it with the radio on and it wouldn't but as soon as I turned it off it started. It will start then die. If I hold my gas pedal down (which I've never had to do before) it will stay running but very very rough and dies as soon as I release the pedal.