I have been running 13.04 pretty successfully on my Lenovo T430 (recent machine - purchased in Feb 2013, Core i5-3320M CPU, 8gb ram) for many months.
I decided to upgrade to 13.10 over the weekend, and since then my machine has been unstable in a number of ways - the most annoying is that the UI has been completely hanging multiple times a day - at least once a day since then, often 2 or 3 times a day. The hangs usually involve the GUI locking to the point where the mouse still moves around, but nothing else works - and I have to go to a virtual console and kill the gnome-session. Twice it has hung to the point where I can't get to a virtual console and have had to hard-reboot.
I don't know if it is related, but I've also noticed that the 'Ubuntu Software Center' seems to crash immediately every time I try to start it.
I have installed a few apps (like darktable) from separate PPAs, but I don't believe I've installed anything 'core' from these systems. The one 'abnormal' thing that I had done under 13.04 was to switch to the gnome UI instead of Unity (after Unity stopped working for me after a 13.04 update).
I did do a clean install of 13.10 on a separate partition and then installed the gnome UI, and I saw the ubuntu software center crash there also - although I haven't run it long enough to see if the UI hangs happen. I also tried installing the 'Gnome Ubuntu' pre-built image, and at least ubuntu software center worked.
This is my daily machine for work, so I need it to be stable. I'd rather not go back to a clean system and set things up from scratch, but I can if necessary.
My current plan of attack so far is to start with the clean 'Gnome Ubuntu' image and rebuild things - although a way to re-stabilize my existing system would be a big improvement...
Any advice on better ways to get back to a stable system?