Any Ideas? A3 Auto, surging during warmup

Would welcome any comments - my 1998 A3. 1.6 auto occasionally "surges" forward whilst I'm slowing down in a queue of traffic. Putting my foot hard
on the brake stops it, I then take it out of drive, put it into neutral and the revs shoot up to about 2500, knock it into drive again and its all ok. It only happens during warmup, never happened at normal temp. Also, a couple of days ago, whilst idling at some lights it stalled. It started again no problem but its never done that before. Any ideas much appreciated. PatC - UK
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