1996 Volkswagen Polo starts then immediately dies

started as an occassional blip, but now starts and dies after approx 1-2 seconds regularly like it runs out of fuel, have had it put on machine which said sensor in carburetter needed changing, it still starts and dies though garage seem a bit stuck as it is an intermittant prob that is getting worse, some days it works perfectly though, has done this hot and cold getting very frustrated any help appreciated