problems with file permissions

Im running OS X 10.5.8

I have an Apple RAID with 2 disks mirrored. I have a share with lots of files, about 1TB data.I just noticed that the permissions on some directories/files are totally wrong. I dont know how they became like that. Now when i try to remove the permissions to add a new set of permissions i cant remove the existing ones. I cant select a user/group and then click the - sign below. I can add new users though. Strange thing is that there is a group there which has Custom access which i want to change to read/write but i cant even do that. Anyone have any ideas on where to troubleshoot?