Have a look through this forum thread. It might help
http://www.chess.com/forum/view/help-support/tactics-trainer2Tactics Trainer Rating Bug
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For some reason, it seems that my tactics trainer rating was reset to my highest rating last week. Looking at the graph and adding the values, this does not appear possible. I clearly remember being around 1300 or 1400, yet I was reset to around 1800. I solved a few problems since then before noticing, so I am now a bit lower that where it seems to have been reset.