MEPIS at one time was based on Ubuntu, but the MEPIS devs went back to a Debian base because they claimed the Ubuntu base wasn't stable. Here is the default MEPIS /etc/apt/sources.list while this distro was based on Ubuntu. Now, I'm no expert, but I have always been told to never mix repositories because it can make the system unstable.
# See sources.list(5) for more information
# This file should be edited through synaptic
# Do NOT use Ubuntu restricted pools with MEPIS!!!
# MEPIS improvements, overrides and updates--the MEPIS magic
deb http://apt.mepis.org/mepis32-6.5/ mepis main
deb http://apt.mepis.org/simply32-6.0/ mepis main
# Ubuntu packages mostly compatible with MEPIS 6.x
deb http://archive.ubuntu.com/ubuntu/ breezy main
deb http://archive.ubuntu.com/ubuntu/ dapper main
deb http://security.ubuntu.com/ubuntu/ dapper-security main universe multiverse
deb http://archive.ubuntu.com/ubuntu/ dapper-updates main
deb http://archive.ubuntu.com/ubuntu/ dapper-backports main
# Unsupported somewhat compatible packages from Debian and beyond
deb http://archive.ubuntu.com/ubuntu/ dapper universe multiverse
deb http://archive.ubuntu.com/ubuntu/ dapper-backports universe multiverse
# Extra packages not available elsewhere
# deb http://medibuntu.sos-sts.com/repo/ dapper free non-free
This sources list mixes Ubuntu Dapper, Ubuntu Breezy, Ubuntu Dapper Backports (not officially stable), and MEPIS repositories. Can you see why this would make for an unstable system? The instablility of this release of MEPIS was not due to unstable Ubuntu repositories, rather it was due to the MEPIS developers mixing repositories.