Why does my sewing machine keep breaking the thread?

My Beaver 9 Mk2 is driving me up the wall. It is so frustrating because it can work perfectly for a period and then, for no apparent reason, the thread from the top spool will break and continue to do so over and over and over. Can you advise please. I am tired of this temperamental sewing machine. Interestingly, one of the Oxford dictionary meanings for 'temperamental' goes as follows: working erratically and inconsistently; unreliable: a temperamental sewing machine!!!!!!!!