leak
I agree with mopar1968. Sounds like a leak. Have you checked the linkage at the butterflys. Sometimes they get worn and start leaking thru the pivot. One way to check is to spray a little WD40 on the pivot while it's running. You can spray around the base also. If it changes the idle it's being drawn thru the pivotor a leak. Or try choking it partially off with your hand, if it picks up rpm , it's getting air from somewhere.