oil temperature guage problem

[sash]

For some reason the oil temperature needle on by 2000 S4 will all of a sudden drop to the bottom end of the scale. Sometimes, after driving for a while it will slowly rise back to the correct temp. Its seems to be quite random, sometimes it does it, sometimes it doesnt. havent noticed the performance affected in any way.
Where about is the temp sensor for the oil located, is it easy to get to?
Anyone have any ideas to what may be causing this?