whenever my car gets around 220 degrees,it idles below 1000 RPms,only when im stopped,it runs fine when im cruising but when im at a red light or im stopped it will go to about 500 RPMs and then back up to 1000 then back down and up,i replaced the ignition coil,but nothing,when i first start it,it runs fine,idles good,but when it gets hot it does that.any help would be greatly appreciated.

Diesel