Need some connectivity troubleshooting help

[Protoform-X]

One of my clients is having a problem with his network where every 15-30 minutes his client applications that are hosted on his servers will lock up. The shares seem to be down when this happens, I can't access them. Ping to the servers keeps working. Any network drives change their status to "disconnected." The switch is a Dell managed switch and has not logged any errors.

I'm stumped right now. What should I check next?

Forgot to add: I have replaced the switch with a brand new one, didn't solve the problem. Even the connection from Outlook to Exchange is dropped during these times.