Samsung Galaxy S3 keeps tuning itself off ! Help ?

[Will]

So I've had this S3 for exactly 1 year. A few weeks ago it started to turn off by itself unexpectedly. Randomly, sometimes when speakimg on the phone, sometimes when on Facebook, sometimes while listening to music. I observed that it used to download apps by itself, but after I got an anti-virus, it stopped. It annoys me so much. When it turns off, the Samsung Galaxy S3 logo appears, dissqpears, then appears again, repating this for 100000000 times. Only by pressing all the possible buttons it gets out of this appearing and reappearing or whatever. It just makes me want to go for Apple, even though I don't like it so much, any help ? I tried wiping chache, booting in safe mode, virus scan, and even software update, nothing