i had this with my old cossie was just driving along one day came up to a juntion and the revs wouldnt go below 1800rpm disconnected the idle valve and it went back to normal again as soon as i connected the idle valve back up hot or cold it would go to 1800 rpm and it turned out to be a broken wire going to the throttle position sensor as soon as i fixed the broken wire by putting another plug on it was fine.....

so have a look mate might be wrong but its worth a look