I and my friend have been buying expired domains with PR to sell backlinks. We started to buy them the 5th February, after PR update, and we checked they were real PR. We buyed a lot of .info, some .net, .com and 2 .co. Today, .co domains have dropped to PR0, without any cause. This morning they were PR4, so this is not about PR update. Could anybody tell us what happened? Will happen the same with the other domains? Thanks.