My BoulivardC50 2004 is missing when it gets hot.

When i start the bike it ticks over just fine, i always leave it running for about 5 minutes before setting off a journey and its always been perfect, but about 3 weeks ago after i havd travelled about 25- 30 miles, and come to a stop at a traffic lights i would start to get a miss in the tick-over as if the bike was about to cut out, so i opened the throttle a little to increase the revs, and she was fine, but the missing would continue at every traffic lights i would stop at.
I increased the  tick-over but the same thing keeps happing,I am going to clean the filter and change the plugs, 
can I ask your opinion on this .
 Thank You so much
John

My issue was plugs.