engine bucking, at 2000 rp/m

Dear friends
my Punto Sporting MK1 1998 is having a strange problem. When i drive in my town, were we have a speedlimit of 30 km/h, i drive at 2000 rp/m
in second gear and than its if the engine has a problem with the fuel cut off system. I don't do anything with the throttle, but the car starts bucking like it is short of fuel. When you make more revs, say 2500 the problem is over.This problem has shown in all the gears round 2000 rp/m. I know that the fuel cutoffsystem comes in when you take your foot of the throttle and gives fuel at 2000 rp/m. I think the problem is due to that fuelcutoffsystem. Although someone told me it could be, that the throttlevelvehouse is dirty. Has anybody an answer for this problem?
greetings Nico
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