2008 Ford Focus LT 2.0L

[Unknown1]

Hi all, i have a 2008 LT focus hatch 2.0L manual. Ive had an issue for a while but seems to get worse with what the outside temp is doing/ how long the car has sat. I start her up in the AM (driven daily), let it warm up- drive around 3km up the road to the traffic lights, gear down to 2nd, clutch in roll to a stop. Although from clutch in, my REVS start bouncing.

The car will idel around 900RPM-1000RPM (normally) although shes bouncing between 400- 1000RPM for maybe two or 3 bounces then sorts itself out and idel's fine. The other morning the bouncing in REVS was pretty violent. Enough to make the battery light on the dash flicker. Never stalls though.

When its mid day and warmer, same situation at any traffic lights i noticed it does it but more bounces between 800-1000RPM so don't notice it much.

Anyone have any idea what could be causing this? Some one has told me a temp sensor but im not really sure about that, but if im wrong that fine, they mentioned something about a closed loop circuit.