TD4 engine revs Oscillating

I have a slight issue that i could do with help diagnosing. My 2001 TD4 Auto's engine (112000 miles) has developed an intermitent problem where the engine revs will oscillate up and down when idling. It is most noticable when sitting at traffic lights with 'D' selected on the box and my foot on the brakes, I can feel the car pulling against the brakes and then it will go to the other extreme and almost stall (ocassionaly stalling). If i put the box in 'N' i can watch the rev counter needle bouncing up and down (not a huge amount but it is noticable).
There doesn't appear to be any specific time when this happens and its not constant.
My first thought was that it might be something to do with the throttle linkage (fly by wire?).
Anyone got any ideas?

try changin yer fuel filter, un fuel injector cleaner over a long period. . . .