I've replaced the oxygen sensor several times trying to fix weird things. It never helped.
Thank goodness it's cheap enough in the 1.9L. For the 2.1L it's real expensive.
I've learned that two causes of fluctuating idle are ignition time and vacuum leaks, the latter being the more likely.
Look around for broken vacuum hoses and the like.

>>> Jeff Stewart <fonman4277@EARTHLINK.NET> 5/7/03 >>>
Forgive me, but I've seen numerous posts referring to the "Vanagon Syndrome", mentioning everything from high idle to stalling to fluctuating idle. Which is it, or are all 3 referred to as the "Vanagon Syndrome"? My '84 had high idle and fluctuating idle, checking and cleaning the grounds seems to have fixed the high idle, but the fluctuating idle is still there. I have replaced the idle stabilizer, but that didn't do it. Next I am going to try the oxygen sensor, don't know yet what good if any that will do. 
Jeff Stewart
fonman4277@earthlink.net
1977 Westfalia
1974 S. Beetle Conv.
http://www.geocities.com/busnut4277