Daewoo Matiz power loss

Had this car 7 Years so far very good,but last year in very hot weather i lost power never completely cut out still kept going,when the engine cooled i checked plugs and the No1 plug was sooted up,others were ok,changed plug and lead went perfect until this year,when in hot weather exactly the same thing happened again,very slight loss of power,next day started up perfectly,engine ran as normal for 65 miles when caught in very bad traffic jam and same thing happened again,loss of power but still kept going with slight hesitation on acceleration,but otherwise OK.
Any ideas please