Problems saving/editing on shared folders

I'm working with a new ReadyNAS 314 and am having some issues with getting the shares and the permissions to work properly.

Here are the installation details:
1) OLD SBS2003 server and 7 old XP Pro workstations attached to a domain. The server is about to be retired so we want to just have the users access the NAS shares directly.
2) The NAS was setup and we let it attach with dhcp. We can access the NAS as admin using the IP or the name without a problem. We have 3 shares setup that hold duplicates of folders that are currently shared on the server. Copied all the files and folders over from the server to the NAS without any problem.
4) I've mapped 2 of the share folders on each desktop for them to access the files. I used one user account for all of them that has read and write permissions at the NAS. We sometimes have to reconnect with the password to the share--but I've run into reconnection issues on mapped drives before and expect that.

The problem is that sometimes a particular user can save and edit files and sometimes they can't. A couple of the users could save and edit documents but couldn't save files that were pdf from their scanner. One of their common uses is to call up a file from one of the shares---basically a document template in Word and then save that to the second share in a folder for a specific client. Sometimes it works and sometimes it won't let them save it.

Things I've tried:
1) Added the NAS IP under wins on the server network connection. Seemed to make no difference and we really haven't had any problem seeing the shares and files. Only with saving and adding new files.
2) Created a user on the NAS that used the same credentials/username as the domain logon for a particular user. Left the regular NAS user account active that was used to make the mapped drive connections. In one case this seemed to solve the problem. In another user it did not.
3) We originally had it setup so that "everyone" was given read write permissions. It appeared to solve some of the problems when we unchecked that and then were offered the read write option of each of the individual users. Those were then given read write. That seemed to help a bit and some worked OK for a while but now the problem has reappeared on different user accounts.
4) Found how to use the "Everyone" user with read write and then check the box to allow anonymous access. Haven't tried this one yet. Figured to check here first since I've been spinning my wheels for a day now.

Considerations: Since we are retiring the server---I didn't want to connect to the domain. with the NAS. We are also replacing all of the workstations with Windows 7 units but those won't be installed for a few weeks. Want to keep it as simple as possible so that all the users can get to the same files and do their work without having to log on or whatever.

A colleague suggested setting up hosts files I believe on the NAS. I'm not clear how to do that. It looks like I would need to assign the IP address of each client machine as a host. But right now the option to assign host IP addresses is grayed out.

I know these things get complicated--so let me know if you have any suggestions or if I need to clarify. Thanks in advance for any help.

Have a Great Day,
Rusty