Thanks Fatman. In that case I'm guessing I have a defective switch. When I power it up everything is OK and then a few minutes later everything looses connection. VOIP phones report "No Service", workstations can't log in to the domain, etc. When I swap it with a standard non-POE switch everything works fine.
Pinging the switch returns erratic response times (40, 80, 60ms - sometimes no response).
So far I've updated the firmware and reset it to factory defaults.
Is there anything else I should look at before I send it back?