Google Index and Ranking

I buy expired domains and was wondering if this is normal for Google. Once the domian is mine I put up a few pages which are quickly indexed then rank very high on google, but after 1 week (it always seems to be a week) the pages fall right out of the rankings. They are still listed but they do not rank for Google at all. I had emailed once and they said it was normal. I bought a domain and it ranked #4 for Inuyasha music, I was ecstatic, for a week. It just happend again, I click on the search links in my logs and the pages that ranked yesterday no longer rank. The domains do not lose their age, PR or links in this time.
MSN continues to rank the pages very high but who cares, 80 % of my traffic was from google.

 

I've not experienced it myself (because I don't buy old domains), but I heard that Google watches domain registrations, and when a domain changes hand they reset the page rank to 0. If this is true, it might explain what you are experiencing.