aptitude via cron job generates path errors

I'm running aptitude in a cron job to update and perform safe upgrades every 6 hours. The cron command I am running is this:

(/usr/bin/aptitude -y update && /usr/bin/aptitude -y safe-upgrade) 2>&1 >> /var/log/auto_update.log

My mail box is filling up with this:

debconf: unable to initialize frontend: Dialog

debconf: (TERM is not set, so the dialog frontend is not usable.)

debconf: falling back to frontend: Readline

debconf: unable to initialize frontend: Readline

debconf: (This frontend requires a controlling tty.)

debconf: falling back to frontend: Teletype

dpkg-preconfigure: unable to re-open stdin:

E: Sub-process /usr/bin/dpkg returned an error code (2)

A package failed to install. Trying to recover:

dpkg: `ldconfig' not found on PATH.

dpkg: `start-stop-daemon' not found on PATH.

dpkg: `install-info' not found on PATH.

dpkg: `update-rc.d' not found on PATH.

dpkg: 4 expected program(s) not found on PATH.

NB: root's PATH should usually contain /usr/local/sbin, /usr/sbin and /sbin.

I have checked my PATH and it appears fine:

mike@DRAKE:~$ whoami

mike

mike@DRAKE:~$ $PATH

-bash: /usr/local/bin:/usr/bin:/bin:/usr/games: No such file or directory

mike@DRAKE:~$ su

Password:

DRAKE:/home/mike# whoami

root

DRAKE:/home/mike# $PATH

bash: /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/root/bin:/usr/local/sbin:/usr/sbin:/sbin: No such file or directory

Any help, thanks. BTW, running Debian 5