Theres the truth of the matter right there mate, when an RS has idle issues it could be many things! Mine used to do what yours did because it was leaking thru the dump valve. The way to test that, if yours has one, is to squeeze the vacumn hose on the valve and see if it makes a difference. Check all hoses for the smallest of holes/tears etc and make sure that all jubilee clips are tight and on correctly. Its always a simple thing that solves the annoyingest of problems!