Prob. after puttin gas in my 2002 Hyundai Elantra GLS?

I fill the tank everytime it goes empty.. like 1/4 tank o so.. i take the cap off pump gas finish and pay then i try to strt it and its strts but then dies out right away.. I have to keep my foot on the gas and break at the same time to keep it running untill i move like 500 yrds.. WHat the hell could be causin it.. NO IDEA... hmmm.. smoke test n stuff was tested.. it wuz normal.