Technical help needed GT TDI 110

I have recently purchased a golf gt tdi 110 bhp 1999 model , i have noticed that when i pull up to stop ,the revs fluctuate , giving the feeling that it is about to cut out (though it doesnt) , it happens more when the car is warm .

does any one have any idea's of what the problem could be ???

i have read that it might be the ecu , but is this just assosiated to a definate 'cut out' of the vechile ..

which brings me to my next question , if it was the ecu , do i have to buy a VW replacement costing in excess of 800 uk pounds or can any one suggest alternative suppliers .

any help is much apreciated !!!

thanks