Car dying out while Im drivin it

I recently purchased this vehicle. I got a complete tune up and oil change. Well when I purchased the vehicle it didn't have a stereo in it at all. When I got a radio installed the car wouldn't start. Well I have since then taken the radio out and replaced the idle control sensor and the massive air flow sensor. The problem is I can start the car, and drive it down the street to the store and it will die out. After this happens, I will wait to try and restart the car. It starts back up and sometimes drive like there is nothing wrong with it at all other times it will die back out if I put my foot on the gas pedal. When it is dying out I can have my gas pedal completely to the floor of the car and it will not idle back up or anything. I am so confused. I have had severl mechanics look at it and they are at a loss as to what the problem actually is. Any advice will be greatly appreciated. Thanks....