Strange DHCP client behavior

My WAN connection is via a cable modem, and my ISP's DHCP servers appear to use a random lease duration with each renewal, approximately in the six to ten hour range. All well and good, but my UTM appears to be having trouble getting a renewal.

At half the lease interval, it starts sending DHCPREQUESTs to the DHCP server that served its current address, however these go unanswered, so it keeps trying every ten seconds or so for the next three to four hours until the lease actually expires, at which point it sends a DHCPREQUEST to 255.255.255.255, only to get a DHCPACK for the same address from the same DHCP server! The whole sequence then repeats again a few hours later, generating hundreds of unrequited renewal requests.

I'm tempted to write this off as a misconfiguration on the part of my ISP (who have proven themselves incapable of properly running other services in the past), but I thought I'd toss it out here in case there is some further tracing I can turn on so see if the problem really is at their end, particularly since I noticed that there were some changes made to cable modem DHCP in 9.003 (which I'm running).

Here's the relevant messages from the System log for the last couple of days. (The daemon restart at 17:10 was due to me manually disabling and re-enabling the WAN interface.)