I had also error -108 troubles with my presentations and made a lot of tests.
I did 22 seconds presentation made of star wars scroll effect and globe effect. It was impossible to export it in full HD on Mac Air (error -108).
There was a lot of free hard disk space, there was nothing else running, system was restartet etc. Nothing helped.
Finally I went to Apple shop and asked to render my project on bigger Mac and they did.
The most sensible option is export project using QuickTime. When it does not work try to export project without QuickTime and after that transcode your mov with MPEG Streamclip to mp4 which works on most of home and mobile devices (PS3, mobile phones etc.). When this does not work just go to Apple store and ask to export your project on bigger Mac.
It is wery fair solution i think, Apple f... up this soft so they have to export project for me up till they will fix all bugs.
I think (but I'm not sure yet) that the problems is related to Texts we are making in project. Probably some char codes cause wrong memory allocation or something. Some projects that I changed texts were suddenly exported and some stopped.