Accelerator performance

I have encounter some difficulties using accelerator.

On some large windows file servers (4TB+), it periodically runs very slowly (days) as if it is doing a first backup with accelerator. Or if it is an incremental it appears to backup everything.

I am trying to figure out if something is modifying the files in such a way that they all appear changed to accelerator, or if perhaps this is a bug?

When it is working the performance is great. when this happens the backups run for several days.

The netbackup server is a 5030 running 7.5.0.6. The clients are running windows 2008 or windows 2008R2.

Thanks!