The project I am working on is a 720 30Hz.
On selecting three different reder projects, mp4, flv, H264 the entries in the que stay "paused" and the rendering indicator on the right stays at zero. On viewing the folder where the clips are being rendered I can see the files being created one after the other.
I am using an AMD phenome system and the rendering has set my CPU temperature alarm off. Looking at System Monitor, all CPUs are at 100%, never seen that before.

How is your memory utilization? I have seen problems where it skyrockets and the system become unresponsive due to paging virtual memory. Usually, this is a result of a bad MLT FFmpeg (avformat) module and needs to be rebuilt clean (including FFmpeg).