Why does my Alcatel Fierce 4 keep shutting off randomly?

When I use my phone it will work fine for a couple of days and then in the middle of an important task it just shuts off. Anyone know what the reason for the random shut off could be? It's getting really annoying and I'm trying not to break the darn thing. My husband says it might have been an app that I downloaded previously but personally I think it's just cause the phone is a little crappy. Any suggestions on how to fix said crappy phone that is annoying as heck?