Rendering freezes PC

When I try to render projects, the computer freezes part way though on any settings. It has only just started doing this:
Kdenlive 0.92 from the sunab ppa, running on Ubuntu Studio 12.04 AMD Phenom II 965BE at std clocks, 16 Gb Corsair Dominator, 1600 DDR3, Nvidia 460 graphics card. Have tried an unistall and reinstall of Kdenlive and all libraries but nothing is working.