No spark

I am looking for some advice before i spend money i dont have on my duster i went to start it a few weeks ago to go put gas in it and i had no spark so i changed the box and it fired right up i went to the store and put gas in it got back shut it off, a couple of days ago i was going to drive it to work and again no spark so i changed box again but it fired twice before i got it warmed up and then no spark again this time my fuel gauge went up to full and then back down and i dont know where else to start