Sounds like either idle air valve (on top of motor) or idle stabilizer unit
(located behind the passenger side tail light).  Bentley has a way to test
both of these but if you need me to I can fax you the pages.
I have the idle air valve new for $240 (VDO) or $200 (Generic unit)
And the idle stabilizer unit for $199 new.
Let me know if I can help.

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