Possibly idle sensor?

I have a 1998 Mazda 626. I've taken it to Mazda repair and two other shops trying to get it fixed. This has been going on for 2 yrs now. Idles like poop, engine acts like its going to die, worse with air conditioning. Mazda told me I needed a tune up. I get a tune up and it only lasts maybe 2-3,000 miles and starts missing again. Usually when I check the plugs one is misfiring, so I replace the plugs and wires and again it lasts about 2 - 3,000 miles. It never runs great but it is better for a little while.