Stratus "cuts out"

Just got a 1996 Stratus with 2.4 engine; 95,000 miles. Problem: occasional rough idle, rough acceleration--like the engine cuts out. Won't go above 45 mph without cutting out, often accompanied with a noticeable "muffled" backfire sound. Generally weak acceleration, but at other times the engine purrs like a kitten. Car sat idle for a year but has new oil and filter, air filter, etc. Haven't checked plugs yet. Put can of fuel injector cleaner in but no improvement. The car was given to me and I have no experience with the make/model and I don't have any maintenance records. Can some suggest a place or system (fuel, electrical, emissions, etc) to start tracking down the problem(s)? Thanks, Chris