On my Ipad 1, When I am using an app it automatically closes, the screen goes blank and the home screen comes on after about 5 to 10 seconds

I have an Ipad 1. After opening an app and using this for 3 to 5 minutes, it closes, the screen goes blank and the goes to the home screen. you need to click on the app again to open the application and this happens all the time.I have reset the pad, re-installed but to no avail. please can someone help