Hmm... could be a Layer 1 cabling issue. Have you run rapid ping / continuous ping tests connected directly to the cable modem using (one set of tests for each cable).
Also, if all cables are good, try the same tests in the following order:
1. Only cable modem connected
2. Cable modem and router
3. Complete setup (switch)
Also, is your router assigning the switch a DHCP address? If so, it may be a good idea to make it assign a static IP to your switch.