My commuting/cyclocross bike (a Surly Crosscheck) has a problem.

If I tighten the quick release skewer to a normal level, sort of firm to close it, then if I step on the pedals hard at some point, the rear wheel basically comes right out of the dropouts.

To keep the wheel actually in the frame, I have to cinch the quick release down hard enough that I get a bit concerned about ever opening it again. The springs get mangled.

This is a somewhat recent development and appears to be getting worse. Any ideas?