This probably applies to many cars but this is a problem I have with my LS Focus.
I've done 230,000kms in it and in the last 30,000kms or so its had a progressivly worsening idle when it would surge, then almost stall, then surge again, it got to the point where it would stall as you pushed the clutch in when coming to a stop.
The cause of this is carbon build up from the EGR tube coming from the exhaust into the intake, immediately after the throttle body. There is a small gap between the throttle body and the butterfly that keeps the idle smooth, with this blocked, the
ECU tries to compensate by opening the throttle body momentarily when it senses its about to stall.
To fix this, take the airbox out by undoing the clamp, pulling off the silicone band at the front of the airbox (closest to the front of the engine bay), and undoing the rocker cover breather tube at the bottom of the airbox, then by pulling the box vertically.
Unplug the throttle electricals by sliding the red tab towards the passenger side of the car and push it off with a small flat head screwdriver, then undo the 8mm bolts on the throttle body. Note that there is a bracket holding cables on the right 2 bolts. Be careful that no foreign objects fall into the intake
When you have the throttle body off the car, clean the insides with carbie/throttle body cleaner and a toothbrush, really get in there.
Replace all the parts and enjoy.