Cassandra goes down after some some days
This seems to be an issue with the values for column_index_cache_size_in_kb and key_cache_size_in_mb in your cassandra.yaml file. As explained here, the database will stop when the total size of all the index data for a partition is greater than the value defined in column_index_cache_size_in_kb.

If the tuning of these values don't fix the issue, you may consider using larger instances for the nodes