I keep seeing seemingly-random XP Pro PCs that have issues accessing Netware (6.5 SP4) drives (mixed MS and Novell environment).  Novell login appears OK and the drives appear in My Computer but when you try and double-click a file in one of them, it just sits there for a variable number of seconds before it makes any visible attempt to open it although eventually it does.  A network speed test using FTP from a Windows Server goes at multi-MB/sec.  The PCs that are affected vary in make and model but here is what I've been trying:

1. Remove currently installed Novell Client and install v4.91 SP4.
2. Remove NICI and NMAS as I've been told we're not using them.
3. Turn off 802.1X authentication on the NIC.
4. Uninstall QoS.
5. Delete the regkeys that search for remote printers and tasks.
6. Check DeviceName regkey points to "\Device\NetwareRedirector"
7. Auto search for shared files and folders is off.
8. Simple file-sharing is off.
9. Netware is the highest listed provider in the search order.
10. UNC Path Filter is On.
11. No IPX installed.

The problem is unpredictable as sometimes it will seem to go away and then come back or I think I've found the fix (eg removing QoS appeared to fix 3 PCs) and then that fix doesn't work elsewhere.  I also think a lot of users are just putting up with it and not contacting us.

I've also compared the output of SLPINFO /A  from a misbehaving PC to that of my own and they are identical.

Any ideas?  Apologies in advance if this will take some time for me to troubleshoot.
Thanks for your help!