static IP & DHCP problems on LAN

[Adam Funk]

I had some trouble with my NAT router the other day, so I was using
the cable modem (which I normally use in "modem mode", i.e., straight
through to my own router) for a few hours.  I got everything back to
normal, but on two consecutive days now, around 01:30 or so, the
machine (desktop & home "server") has gone off its static IP address
and onto a DHCP address from the router, which makes it impossible for
me to SSH into it (unless I can guess the IP address correctly).

When I sit down at it & run

sudo ifdown -a
sudo ifup -a

it comes up with the correct static address, but then something funny
happens early in the morning to turn it back to DHCP.  I also tried
running the '/etc/init.d/networking restart' command today, & that
screwed up graphics & lightdm to the extent that I rebooted the
machine in desperation.

Q1. How can that init.d/networking command screw up things unrelated
    to networking?  I expected it to do little more than 'ifdown -a ;
    ifup -a'.

Q2. What is allowing something else to override my static config in
    /etc/network/interfaces in the early morning?  The config file is
    the same as it was before except for a commented out stanza.

Thanks,
Adam