OS 10.5 - Can't ping local network hosts after a few hours from reboot

So this is a really really strange problem.

I have a Core 2 Duo iMac which I use as my main machine, a MDD server, and a macbook laptop. The iMac and server are both connected via wired ethernet through a Dlink DGS 2208 gig-e switch. The switch is also plugged into a linksys router, which is connected to the internet as well.

So the problem is this: after a few hours from being turned on or rebooted, my iMac suddenly loses the ability to ping the other hosts on my local network. I get "No route to host" errors when I ping my server, my laptop, or even the router itself! Despite this, internet access continues to work without a problem (so even though the router can't be pinged at its local address (172.18.1.1), it's still correctly working as my default gateway). I also get a "No route to host" when trying to ping the subnet's broadcast ip.

At first, I suspected it might be the switch, but the problem remains regardless of which port I'm using, and only goes away with a reboot of the iMac. The laptop and server remain unaffected, and can ping each other without problem.

I tried adding a static route back in, but that seems to make no difference. I've also confirmed that entries exist in my arp table for the other local network hosts with the correct MAC addresses - even when the iMac is unable to ping the server, it still knows the server's MAC address in its arp table.

I've tried dumping the routing tables (using netstat -r) when the iMac is working and comparing it to when it's not, but, although the server tends to appear when the iMac is "working" and not when it's not, adding back a static route makes no difference. Also, any other hosts on my local network (like my roommate's laptop) which are listed by netstat when it's not "working" are also unreachable - ping returns the same "No route to host" error.

I really can't figure this out - any advice on things to check would be appreciated. I don't want to go in and reinstall on such a small thing, but it's really annoying when you can't reach your server unless you reboot every 5 hours.