Old Dodge Caravan runs rough

My 2000 Dodge Grand Caravan, sport edition, has 228,700 miles and is running rough intermittently. The spark plugs and wires, coil, fuel line and fuel filter have all been replaced but the problem of it missing or stuttering is still there, most frequently when I am sitting at a stoplight or going about 55 mph. (If I go really, really fast it runs really, really smoothly so that is how I have been coping with it for now.) I don't want to keep replacing parts and have it still running similar to someone who has a heart rhythm problem. What do you suggest?