Cyclic Redundancy Check is CRC, which is a method of verifying that data is correct. If you get CRC errors, that means your data is getting corrupted somehow.
The client is sometimes the culprit. Try removing a torrent on your current client and adding it to another client. Wait a couple days, and do a force re-check. See if the files still show 100%. Do this a few times. If any of the torrents end up at less than 100% after a while, it's probably not your client's fault.
If it's not your client's fault, the next best guess would be that your hard drive is going. If you have multiple drives installed, you can try moving some of the CRC failed torrents into a different drive and see if they still fail after a while. If it turns out to be your hard drive, back up all your data on that hard drive as soon as possible, because that drive is going to die soon, and once it does give out there's no guarantee you can get anything out of it.
If it's neither the client nor the drive, there's something weird going on behind the scenes.
