Car won't stay on if we aren't pressing he gas pedal.

My mom has a Toyota Avalon and it has problems staying on. It will turn on when I turn the key but it turns off the moment I let go of the gas pedal. Before, we used to warm up the car by holding the gas pedal for a few minutes till the car would warm up, and then when we let go of the gas pedal it would start. But this time it will not stay on if I let go of the gas pedal. If it does happen to stay on, the moment I use the brake to slow down, the car turns off. The radio and electric things stay on but not the engine. Thank you for you time. Anneth