1993 Ford Probe Spitting and sputtering

Hi. My car ran great when I first got it about a year ago. It started to stall only on occasion when I put in my clutch, now it doesnt stall at all, it spits and sputters like it cant get going, jerks and backfires. I cant figure out what the problem is. I have had a recent oil change (they put in a diff oil than I used) and I did a complete tune up not that long ago, except the fuel filter. Also since this started, my gas milage has gotten about 1/2 what it used to.