car idelin to high or diein

got my a3 n its a great car but recently its been diein on me as if its runnin out of petrol when its got plenty then it starts again with no problem after about 5 minutes or it wants to idel at around 3 or 4 grand its after it went in the garage the first and they said no fault i carried on then it happened again its now back in the garage n they say its my temprature sender

any suggestions???