Higher Than Normal Revs When Idling

[teekay523]

My car recently has been idling at 1500rpm at a traffic light ,when braking it goes all the way down to about 800 then when the car is stops moving it jumps back up to 1500rpm. also when starting up the car when cold , the car judders constantly like its about to stall and its about 700-800 revs... anyone have any reason for this? have looked at wether air con is swtiched on etc but all the systems are off.

Tim