"Operation Cannot Be Completed Because Item (xyz.xyz) is in use" - NAS Copy

Hello,

I have a MacBook Pro running 10.5.

It's connected on a Linksys network that also has a few PCs and a Infrant NV+NAS.

When I copy files FROM my Mac to the NV+, I'm often running into weird problems with :
a) Being unable to copy the file at all
b) The file starting to copy - making it about 99.9% complete and then giving me an error of "Operation Cannot Be Completed Because the File XYZ is in Use".

Now:
- The file is definitely NOT in use.
- If I try and do the EXACT same copy (actually copying it from another network drive to the same location) on a PC running XP, it copies fine

Few other bits of troubleshooting:
- I've actually plugged in a few other NAS devices (including a USB drive that plugged directly into the Linksys Router I have) and I was having very similar issues
- I've tried doing the copy on wireless (which is what I"m typically running) as well as connecting the laptop in via a wired connection. and have the same exact problem.

Often, if I try it a few different times, the same thing happens. Sometimes it works complete.

Sometimes when I copy it to a different directory on the same share of the NAS, it works. Sometimes not...

I also run my Timemachine via the NAS - sometimes it works fine, sometimes it craps out in the middle and stops..

I have NO idea what the **** is going on except that I know it's the MAC...

I talked to someone at Apple and they thought that it might be the Router.. Maybe.. I had a Linksys WRT54G router and had the same problems before I upgraded to another Linksys N Router (same issues)..

ANY thoughts? Any things I can look for? Not even sure what I can post except that I know I"M not the only person that is running into this issue.

Eric