A couple more additional comments to add to my post above:
(1) Quick & dirty solution to out of memory error is to select a few clips at a time and render those only (not all the clips in the sequence). This does work from time to time
(2) Apparently gremlins do get introduced to sequences as they get more complex over time and certain aspects of the sequence may get corrupted. To fix this:
(a) start a new project;
(b) create a new sequence:
(c) select all clips in your existing sequence and copy the clips from your problem sequence into the new sequence in your new project;
(d) set your scratch disk to your existing hard drive or additional internal hard drive (dirt cheap now for 1+ TB); and
(d) render without any trouble.
If this still does not fix the issues, google a document called Final Cut Basic Troubleshooting
Hope this helps.