sounds electrical.
Change plug and check to see if it is sparking constant. Possible something is giving out now and again.Only way is to change bit by bit or get someone to check electrics. if it gives out now and again this will be hard to find. I would change plug HT lead to start.
If you cleaned carb and then doubtful it is this. I would not work at all and would be a constant problem.
BB
April 2009