Alain
Sounds liek a small vacume leak, or perhaps the oxegen sensor is bad .
Check the milage since the ox sensor was changed If it's over 60k miles thats
agood bet .This is known as a "hunting idle". As for the possible vacume leak
remove the rubber "s" shaped tube that connects the air box to the throttle
body .They tend to crack . Also look at the small braded rubber vacume hoses
(about 5or 6 mm in diameter) that run all over the engine compartment.
These are only some possibilitys

mike
85westy 213k