What engine type/size do you have?
On NA Zetec models, the tickover (idle) is controlled by the PCM (Power Train Control Module) via the IACV (Idle Air Control Valve). If you think you can adjust the idle with the old carburettor style throttle plate screw butting up against the throttle body stop, you're mistaken; the PCM will compensate for any adjustment you make.
Think of the IACV as a throttle plate bypass passageway whose cross sectional area is adjustable. When the throttle plate is fully closed at idle, the PCM varies the size of the passageway to obtain the desired engine speed and monitors it based on other engine sensors. It's claimed to be more accurate than the old method.
If you are having erratic idle problems, the two most likely candidates for problems are the IACV itself or vacuum leaks after the MAF (Mass Air Flow) sensor.