Engine idle miss, erradic

I have a 2003 Tribute that has an idle problem.  Occasionally the idle runs rough, you can feel a lite shake in the car (almost feels like it's missing a cylinder).  I recently (2 months ago) had all the plugs replaced due to starting problems, inermittantly missing a cylinder.  About 1 month ago I had all the coils replaced and the plenum cleaned because of a steady loss of cylinder.  A friend of mine says it could be the harmonic balancer, don't see how that could be unless it's loosening up itself.  Any suggestions? Thanks

if its a v6 these cars have a comon problem with the idle control valve.

My 01 is pulling the same crap, except its not just when it sits, and it does not have this problem in the early morning only when it gets hot out.  I replaced all the plugs, but not all the coils (just 1 coil, too damn expensive $99.75 for 1). Took it to a machanic and he said just find the orginal plugs, buy them change out the wires for the plugs. Hopefully it goes away, (do the cheap things 1st then see if that solves it if not replace all the coils). Could there be another thing making this happen? I am going to sell the tribute soon but I would like to have it fixed 1st.

Thanks for the advise Jim.  I'll take a look at it this weekend.  Is it possible that the ICV only needs a cleaning? or do you think replacement is necessary.  Thanks.