IT KEEPS CRASHING EVERY 20 mins ! please help !

Put the summary of the bug you're having here

What I expected to happen was...:
it crashes like every 20 mins. even when i am playing by my self. :/ online is jaggy as like unplayable lag.

Steps to Reproduce:
1. I turn on launch Minecraft
2. everything works fine
3. the randomly it crashes .

i dont know why it happens my computer is not shit btw ... can you help meh fix it ???!!!