winlogon.exe hangs causing 100% CPU

On a corporate network we support was have 26+ Terminal Servers, running on NT4 server & metaframe 1.8 They have all been working fine for over 5 years+. Recently a few servers have started to experience a problem where by the WINLOGON.EXE process takes up to 100% CPU and the server performance degrades to an unusable level. The only way to resolve this problem is to reboot the server - not easy when anywhere up to 30 users are using it (might not seem like many but it is to the client).

All servers are service packed to up to SP6.

We have tried both of these numerous times:
http://support.citrix.com/forums/thread.jspa?messageID=64357&fromSearchPage=trueﭥ
http://support.microsoft.com/kb/243840/en-us but they haven't helped.

We have already gone down a few routes namely:
Printer drivers problems - hasn't help
LMREPL.exe & Directory Replication. Some (at my company & client (all Techies)) believe this may be the problem - hasn't helped.
Profile rebuilds. We have rebuilt one sites profiles but the problem is still occurring.
Spy ware etc. All servers are clean as a whistle after running spybot and AdAware.

This is causing the client more and more problems as well as a lot of work for me & my team. There is an upgrade for the client to Win2k3 (software and hardware) coming but the client is talking about postponing until the above problems are resolved. As much as I want to say to the client just upgrade we all need to understand (and try to fix) the problem before the upgrade.

Any help would be great!
Thanks
pappaslim