Juan
I take it there's no obvious difference between the clients and they're all using the same version of Outlook? Also, on the bad PCs can you telnet to port 135 and 7575 to ensure this works?
I take it if you go direct and bypass the f5 it works?
I'm wondeing if it's a TCP issue and possibly keep-alives? Can you double check what your TCP profile is set up for it's Keep Alive settings, compared to the deployment guide?
This askf5 article may help: http://support.f5.com/kb/en-us/solutions/public/8000/000/sol8049.html and this MS article: http://support.microsoft.com/kb/2535656
Rgds
N