I'm pretty sure Trueblood's problem is that the tire is not gripping the rim properly. It's a combination of an old straight sided rim, a new tire made for hooked rims, and high pressure. Possibly the tire wasn't seated right; no way to know that now. But if a tire has blown off a rim once, I tend to expect it to happen again.
With lower pressure, I hope not. The panaracer tires are 27x1 1/4, presumably manufactured for the older standard of wheels. My question is, what tire WOULD be designed for straight sided rims if not the panaracers? I have been careful in seating the tube and tire.