1998 eclipse gs idle trouble

hello i just bought a 1998 eclipse gs and it was idling between 700 to 800(not steady would go from 700 to 800 in a sec then back to 700, so i change the plugs and wires along with the oil and filter and it ran at 1200 rpms steady for a few days and now its idling between 900 and 1100 rpms(jumpy) any help?