Thanks everybody!

Before I even read the responses here I tightened the wingnut (seemed intuitive), and on Sunday's ride didn't experience the problem. (On the other hand, Sunday's ride didn't have nearly as many steep hills as Friday's...) Sunday night I cleaned & lubed the cable guides under the bottom bracket; now if the weather ever gets decent this week I'll take it out agin, mash up some hills, & see if I'm in the clear.