This is a "where do you start" problem.
So it's either fuel or spark.
Fuel could be old? or there may be water in the tank from condensation, which is getting drawn through occasionally.
Spark might be an intermittent fault such as HT leads, plugs, or anything electronic in between. Experts will tell you that electronic systems either work or they don't, and yet I had an intermittent fault on a module which drove me mad until that was the only thing left untouched.
