According to this site, another chat blog, featuring about halfway down this page:
http://gravityloss.wordpress.com/2008/01/23/hypersonic-cruise-for-the-v-prize/
The Bomarc they discuss disintegrated at Mach 6 after passing Mach 5.5 at 40,000 ft.---and it was still accelerating.
Those particular ramjets were said to be designed for only Mach 3.5. Maybe kind of embarrassing to both engineers and top military brass at the time?