Erratic tickover

[arthurs]

Hi I am having trouble with the tickover on my S3C (decatted) sometimes on idle the revs suddenly shoot up to 2000
rpm if I turn the ignition off then immediately start it again the revs are back to normal.It makes the car tricky to drive as it will happen anytime.The butterflys are almost closed as that was the only way to get the idle at 1000 rpm when I got the car idle speed was 1200 to 1300 so i adjusted the butterflys. Love the car by the way any ideas?
Regards Arthur