Andy,
It could be either. I have a Win98SE box that occaisionally forgets how to find other machines on the network. Reboot fixes it. If it is something with Win98 hopefully a Windows guru can help out. What are the error messages you get when you run NET USE or NET VIEW from a Win98 DOS window?
If you have to restart smb, it could be a configuration problem - but then it should not be intermittent.
If you are using User level security, I would suggest looking through /etc/samba/smbusers and ensuring that either your Windows user/password combinations are also valid Linux user/passwords for your RH6.2 box, or that the aliases are done correctly. Then look through smb.conf to ensure that the Windows user receiving the error is allowed to access the share in the first place (does it always happen with the same user trying to use the same share?).
If you are using share level security, and 'security' isn't an issue (not critical files) try setting the shares on the RH6.2 box to public = yes and see if your problem goes away.
Hope this helps, just a shot in the dark really.
James