Are all the systems involved with this problem running up to date Microsoft patches? When did this start happening? Which server handles File and Print services on that network? If this is something that just started then what's changed? Have the servers handling DFS or File and Print had the "Server" service restarted recently...this will also cause you to get prompted to restart a few services like DFS and Netlogon which may help depending on your topology and the actual problem. I would also check to make sure that there's not some backups going on in the background that are chewing up network/cpu from the servers. Also, have there been any virus problems in the network? Good luck!

Try running Microsoft Baseline Analyzer this will outline errors, vulnerabilities etc.

I am experiencing similar problems. Also, if neccesary try recreating the shares.
Good luck