X, there are many reasons why you can go up and down...
I will assume that the Yahoo listing is a reflection of Google..
Changes in google often occur.. When you first get listed.. Googlebot finds your page.. does a quick assessment and slots you in its results. This can often be a good listing..
Then over time.. 1-3 months, Googlebot assesses your incoming links, your site, page structure etc, and then starts to apply them to where you land in the results...
Normal progress for a new site IMO...
A new site of mine built for specific keywords followed a path like this..
1st week - #1
2nd week - #3-5
3rd week - dropped to mid 30's
4th week - dropped to mids 100's
5th week - back in top 10
6th week - back solidly at #1
This sort of pattern is what I see now with most new sites...
Things that might influence this pattern include,
- where you get the sites linked from
- the relevance and PR of the places you get it linked from
- your keyword structure
- any change in your page
- and a multitude of other things
Same forward plan applies...
- gain more incoming ranked,relevant links
- be specific in the syntax of links used to link TO your site
- be aware that little changes in your page can make a big difference to word structure, density, relevance.
HTH
DD