A few possibilities here:
1. Bad idle position switch (located on the throttle).  Check this per
Bentley manual.
2. Bad idle stabilizer unit (replace with a known good unit or check per
Bentley).
3. Bad idle stabilizer valve (replace with known good unit or check per
Bentley).
I have all of these items available if you need a new one.  Let me know.

Thanks,
Ken Wilford
http://www.vanagain.com
John 3:16
Office (856)-765-1583
Shop (856)-327-0027
Fax   (856)-327-2242