car shuts off every 10 min or so...

i purchased a 2002 a6 quattro with 140000 miles. it ran great for the first two months. one day i went to start it and it just wouldnt even tho it was getting spark and fuel. i waited twentry minutes and it started right up. this happened once or twice a week for two months...recently it has progressed and only runs for ten minutes before shutting off even while driving. i replaced the fuel pump and filter and the problem persists. the check engine light was on due to a catalytic conv issue but no other warnings pop up. i have no idea where to look next. i was told its my security system cause my key fob doesnt work, or my intake is leaking. i am an idiot with cars n have no idea, please assist...