In the last couple of months I made some tests with expired domains as money sites. They had good metrics (PR4+, TF20+, non-commercial sites before, not a single bad link). As expected they all ranked instantly quite well for mildly competetive keywords. Far better than new domains would. However, I found it quite hard to further improve their rankings from then on. I literally built link after link and nothing happened - the same type of links that work like a charme for me in my niche otherwise. What is your experience with that? Any tips? Is there also a sandbox for repurposed domains as for new domains?