write access occasionally lost

We have a set of folders on a Server 2003 Standard box that occasionally will
not allow a certain group write access. It seems to only be a certain group
or groups, and it is at random times. As far as we know, no one has gone in
and changed anything either before or after this occurs.

Any ideas would definately be appreciated,
Thanks

I neglected to include the error message:
"Cannot copy <file name>: The operation completed successfully".

Thanks again in advance