md5 sum mismatch error when restoring in CWM - Perm fix? topic

I'm using Nandroid and ROM Manager to perform backups, but when I boot into CWM, and choose any backup created by these apps, I get the "MD5 Mismatch error". I've seen plenty of threads how to fix one back-up file, but why does this keep happening? Is there a permanent fix?

Thanks