IMac Airport not following thru on DHCP

Question

Client has an iMac 20" Early 2008 running 10.8.4, connected wirelessly to a full-featured router (MikroTik).
Every so often, the router starts generating error messages in its log:
    dhcp, warning: dhcpserver1 offering lease 192.168.10.105 for 00:1F:5B:CB:FD:FE without success.
I believe this is saying that the iMac is requesting an IP address, but then not following through in accepting it.  (There is a four-hour lease time on the DHCP pool, to keep the lease table somewhat more representative of the number of devices actually connected to the router at any given time.)
For example, at 2/12/14 08:45 the router logs one of these lines every 90 seconds until 09:16.
Eventually, the iMac DOES get online.  In fact, the client did not even report the problem -- I found it examing the logs for another problem.
I have dozens of clients with Macs using the same model router in the same setup, but this client is the only one whose router is complaining; furthermore, the iMac is the only device in his house (out of five) that is generating these error messages.  Therefore, I have to assume the problem is some funkiness in the iMac HW or SW.
Anybody have any ideas as to how to get this iMac to stop dropping DHCP on the floor?

Answer 1

That's an interesting feature I've never played with -- and thanks for teaching me something new today -- but it doesn't seem to be relevant to the problem.
If I put something in that field, the router dutifully reports it under Active Client ID.  If I leave that field blank, the router reports the wireless client's MAC address under Active Client ID, at least for every Apple device in our house (there are seven -- only the TiVo shows a blank field).
So in no case is this field ever blank in the router for an Apple product, and on a device by device basis, it is always constant.  Therefore, just changing the value of this field from one constant to another constant is very unlikely to affect the observed behavior.
Furthermore, the issue here is not whether or not the iMac continues to get the same address it once had -- it is why whenever it gets around to asking for an address, it often fails to follow through with accepting it. 
I should add that the client household has multiple Apple devices as well, and only this one iMac generates error messages.