You'll find all sort of suggestions on the forum if you check 'heap zone' problems, such as restarting the computer, making sure no bad characters in the title, turning of Time Machine, etc. All works sometimes, none worked all the time for everybody.
According to an Apple iMovie specialist I talked to via phonecon, something in some file has gotten corruped -- a 1 changed to a 0, or vice versa, and his advice was 'Just re-do the whole project'!
What I actually did was this:
-I duplicated the project twice.
-I deleted the last half of the first project, and the first half of the last project.
-For each I then chose Share> Media Browser, which is sort of like 'rendering' the parts.
-Then I exported each half with Quicktime.
-Next, I imported each half, and put them back together in a new Project.
-Then Share>Media browser.
That worked. What happens, it seems, it somehow the 'bad' linkage was broken in the process, and that done no more 'heap zone' errors.
Hope this helps.