Hi, wondering if anyone can point me in the right direction.

Car has an intermittantly occuring issue, ill try and explain it the best I can, but bear with me, as it doesn't do this all the time;

Car is a 2002 Focus 1.6 Chic, 1.6 zetec engine.

When the car has warmed up and its at tickover, sometimes the engine can 'bounce' between 750 rpm & 1250 rpm, the rev counter needle will fluctuate between the two speeds, for about a minute then settle down.
This happens after the car has been driven and usually occurs when stopping at traffic lights. its goes like this;
sometimes, on approach to a set of lights you dip the clutch, the engine revs can fall to the point when it feels like the engine will stall (circa 500 rpm), then goto normal idle speed (750-800rpm), then start bouncing between 750 - 1250/1500 rpm.

When the engine is under load, eg, at night, with the lights on, the headlights and speedo lights etc will flicker when the engine is at tickover speed.

The car drives perfectly, the alternator is fine and is charging the battrery. its just that occasionally it'll perform the above weird trick.

any ideas as to what could be causing this would be appreciated.

Thanks in advance

andy