FOSS gamedev and creative worlds

OS : Linuxmint 15 KDE
Minetest Version : 0.4.8 (minetest)

When i play minetest, it run normally. But sometimes when i pause the game (esc), and i run another program such as mozilla, libreoffice, when i back to the game, minetest lag and show fps 1 - 6. When i restart my OS the game is run normally with no lag.

How to solve that?
Here is my debug.txt. i can't post here it has 3000 lines.

https://drive.google.com/file/d/0BzhC_dG0DEqtQjRnazh4bjRhcVE/edit?usp=sharing

Maybe you have not enough RAM to run your browser and Minetest at the same time.

No, it sometimes happen,,
My RAM is 2 GB and My Processor is core i3 2.3 GHz..

i guess the process of minetest left on system even i have close it and i don't know how to kill that, the only solution is restart my PC and it run normally,,,