[dhclient] connection closed, exiting

I have a Sun Ultra5 as my home gateway.
After upgrading to 6.0 from 5.4 I started getting problems with
dhclient (I know it went from v3 to v2 imported from OpenBSD)
If my cablemodem dies without signal or something of the sort dhclient
exits like this:

Dec  8 18:38:24 ultra5 dhclient[198]: connection closed
Dec  8 18:38:24 ultra5 dhclient[198]: exiting.

Some cablemodems reset the ethernet interface on their bootup or sync
sequence but since the machine is connected to a switch I take it it's
not an interface connection we're talking about.
On 5.4 I could kill the modem for hours and dhclient would stay there
(as it should) trying to get a lease.
On 6.0 I just have to silence the modem for a few moments and dhclient
gives up and goes away. I couldn't find a knob for this on the manual.

Sometimes it gets even better, when dhclient has saved (but already
expired leases) and uses them setting the IP on the interface, but
since the IP is not valid it doesn't work. And it stays with that
lease until I restart it and it gets a valid one.

I had signal problems the last few days and the leases db it's like this:

ultra5# grep lease /var/db/dhclient.leases.fxp1 | wc -l
      10

How can I:
- tell dhclient not to exit.
- tell dhclient not to use expired leases, or not to use saved leases at all.

--
Joao Barros