depends what you mean by erratic idling if you mean the idle goes from 670 rpm to 730 rpm this is normal and has to do with the ecu hunting for the lowest emissions flickering headlights at night and on the dash is due to the alternator being routed from the battery through a 7.5 amp fuse which causes the alternator to pulse nowt to do with earth points ford sell an upgraded wire which goes from the posirtive on the battery to the back of the alternator to correct this its a well known issue and ford have a technical service bulletin regarding it on the focus and the mondeo the wir ecosts about £18 from ford this will cure the flickering headlights the idle on some cars gets smoother as the alternator is no longer pulsing
qoute from service bulletin Should a customer express concern about flickering headlamps, wavering illumination of the instrument cluster, interior lighting and audio unit the probable cause is an extremely high resistance in the battery wiring harness from the central junction box (under the bonnet) to the B+ terminal on the alternator. To rectify the concern an additional cable should be installed.
The high resistance from the 7.5A fuse to the alternator multi plug causes it to "pulse" so the new loom does away with this by taking a new feed direct from the main battery cable on the back of the alternator. !!