you can get harsh, jittery idle if the timing belt is loose because it
causes the timing sensors to lash.  check the ignition timing.  with the
service connector in place, if you see the timing marks jumping about,
the belt probably needs tightening.

other than that, make sure all the ignition gear is in good running
order - replacement with oem cap and rotor is a good idea, along with
new leads and plugs.  [some high end plug leads, the spiral wound
variety, can be better than oem, but be prepared to pay about double.]

if and only if you've looked into all the above, and it's not fixed the
problem, and you've cleaned the throttle body, run injector cleaner
through it, etc, /then/ you can adjust the throttle body idle position.
 but normally, you don't need to because normal idle behavior is
computer controlled via the e.a.c.v.

Other than what jim beam has said, check your Electrical Load Detector, and
the A/C idle-up.