I provide IT support for several small to medium sized business. One thing I've noticed is that with 2 of the larger clients - workstations will periodically suffer from what appears to corrupt DNS Cache. This is only when resolving non internet hostnames. Each client has several remote sites but this problem occures randomly on both WInXP and 7 workstations.
Symtpoms: "I can't log into Exchange - it shows disconnecte" or "I can't access the AS400"
Solution: ipconfig /flushdns and ipconfig /registerdns
if I do an nslookup, the server resolves the hostname no problem.
I get 1-2 calls that require this fix per month but I can't find a single thing about this on the internet.