A shot in the dark here, but I would try these 2 things to gather more info: (I personally suspect a database issue).
1. Run a repair on the SQl database (maybe also run optimize?) Test and see if issue remains.
2. Copy the database, then import as a second database. Drop and re-create the xlrstats tables in the second database. Test and see if issue remains.
ofc, you will need to be slightly comfortable with sql / phpmyadmin to do this, not saying you have to, but this is what I would do in this case.
If you don't feel comfortable, don't worry about it.