occasional rough idle/misfire

i've got a 90 buick lesabre with 150k on it. occasionally when it is idling, it will run rough and sometimes misfire. This never occurs except for at idle. The plugs, wires, idle air control valve, egr and coolant temp sensor have recently been changed along with a fuel system cleaning. There are no codes, and i have no idea what could be causing this. The oxygen sensor is about 2 years old, and the fuel filter about 1---could one of those be the problem? I have no idea.....