Why did karl marx argue that the capitalism tended toward economic crisis?

I believe that Karl Marx believed that capitalism tended to create economic crisis mainly because of the 'boom or bust' quality of capitalist production so that producing according to the world capitalist market price means that when the price is high ie of commodities like gold and silver and copper then production is increased sometimes very sharply, but then it results in a glut of products  that people cannot afford to buy so the price drops and there is a depression like the one in the 1930's. But there is not just a recession or depression in the 1930's but in the mining industry it usually takes place cyclically every 5-6 years over and over again and when the price is down then it creates intense hardship for people so that they go deeply in debt or get thrown out on the street. 

Ithink a hope that