juddering on acceleration

[phile1988]

right ive noticed the last couple of days my 1.6 55 plate megane has started slightly juddering whilst accelerating, ive checked my spark plugs and although they were only replaced about a month ago all 4 have a black soot on the bottom of them, ive gave them a quick clean and put them back in and the judder seems to have gone for now, my question is what can be causing this as it will probably happen again

the spark plugs are bosch ones and also i have no warning lights etc on dash