djohnston's point about two repositories is well taken - the only second repository that is at all cricket here is the megagames repository you list. Having two "main" repos is a bad deal - slows down your upgrades, and can lead to the aforementioned borked system).
You might try the Synaptic Repository Speed Test. That should tell you which repositories will be fastest for you to use, and help you set them.
Your other option for upgrading is to use
apt-get upgrade
from a root shell. This will upgrade everything that needs to be upgraded on your system, though. (Be aware of this if running low on disk space on the partition where /bin and /usr/bin are stored.)
Synaptic doesn't throw any errors, or segmentation faults or anything, huh? Just vanishes? I remember something like this happening to me back when I was still running PCLinuxOS 2009.1, but the upgrade to 2009.2 fixed that.
Do your symptoms look anything like this thread:
http://www.pclinuxos.com/forum/index.php/topic,96030.0.htmlIf so, it looks like setting a new repository is the way to go, but, if I'm reading correctly you've already tried that. Run the speed test - that should tell you what you need to know.
Good Hunting,
D