Need a bit more information. What is the WAN-side connection on the 4430 to the Internet? Is it Ethernet, T-1, SHDSL, etc.?
Does this happen after a long period of inactivity? It could be that the ISP is expiring a DHCP lease or similar. Try configuring a probe on the 4430 to ping something like your ISP's gateway every 30 seconds or so and see if that helps. You don't need to track the probe with anything.