I didn't get time to stop by the bike shop and pick up cable and housing so I just tore everything down, cleaned, lubed and reinstalled. I didn't see any obvious problems but the guide under the bottom bracket was kind of rough. The cable is bare there so perhaps that's where the problem lies. We'll see what happens now.
If it happens again I think I'll replace the cable with "sealed" system - housing all the way from shifter to derailer. What would be the downside in doing this other than weight of the extra housing? Surely someone makes an adaptor to attach housed cables to braze-on cable ends - what are they called? It's much easier on my pride to have a name for the part I'm trying to purchase when I walk in the bike shop.
