how can i fix my webb diplomat 300?

my mower starts and sometimes runs for a while then stops and wont start again, i check the spark plug and there is no spark, sometimes if it stands for 10 mins it will then start again other times it wont start again that day, i put it away and the next time i get it out it starts!! it may run for 10 mins or an hour, someone please give me an idea!!! thanks.