Running for seconds only is a clue there. I have had one person that had a bad aftermarket car alarm do this. Next is to see if any codes have came up. When did this start and were there issues before hand? Next the question must be is the car stalling as if it ran out of fuel or simply stopping as if the key was turned off.

You need to go to an Autozone or similar store and get a free scan. Could be the fuel filter is very clogged or your plugs are old but you usually would see a decrease in performance over time.

It sounds like the antitheft system. If the key is gray, it has a chip inside of it. Try another key if you have a spare one.

Key or key code might be corrupt do you have a spare key - I was able to start my 2000 LHS with a dodge key from the guy that was selling the car mistakenly handed me - it would start and in about 2 seconds shut off.

does it run if you keep your foot on the pedal?