I can use the computer for 2 hours without a problem, then when I do any random memory hungry action (Youtube Video, starting many application simultaneously) the computer becomes very slow and stays like that even if I kill many processes.
I don't know what causes this and even the Gnome System Monitor shows normal memory and CPU usage.
Restarting the X session doesn't fix it and the only way to get my computer back to normal is with a system reboot.
I'm Using Ubuntu 12.04 with XCFE.
How can I detect the source of this behaviour? And how can I fix sluginess without restarting the computer?