2.1 Dj Bouncing revs? ideas please

[Eta-Beta]

Ok my dj 2.1 engine ,,which is pretty young still,only like 20000 miles has developed up and down revs,,,, basically it will idle at just below 1000 ,and radomly fluctuate up and down , up to 1300 revs and back down again,,, no pattern just as a mind of its own kind of,,,, thoughts please ,ta oh and the engine shakes when it slows off,