mine did this when i first bought it ,i thought it was fuel starvation as it was rough idling and had lack of power ,took it back to garage and they did some checks and came up with nothing so i put it down to a bad batch of petrol ,stuck some of that injector cleaner in it and filled it right up and all has been fine since .yours sounds like damp has got in some were ,buy some of that spray from the garage and give the leads and plugs a good spray.other than that could be a plug breaking down ,when were they last changed