Why does iMovie 08 hang when exporting h.264 video?

Have a 47 minute project editied from HD video. When I try to export it as full HD using H.264, the export gets about 40% complete and then appears to stop. The time to complete increases as long as I don't cancel. Tries twice. Same result. Export using MPEG4 works.

OSX 10.6.8, iMovie 08 (7.1.4 (585)), Quick Time 10.0 (131) on an iMAC with 3.06 GHz Intel Core 2 Duo and 4GB of RAM.