More than a month ago, I bought quite a few (100+) backlinks from high PR (9, 8, 7, 6, ...) pages in a mix of blogroll, guest post, forum comment, profile, etc. But it shocked me today when I find my site's homepage PR was dropped by 1. It's a fresh domain I registered and built from scratch for more than 2 years, not one that's traded on the market. Does anyone have similar experience? How do you explain this and what should I do for the next PR update?