OK, I'm nuts. I had a major tune about 400 miles ago. They changed out the rear cog. It was fine for about 100 miles and now it won't stay in a single gear - and shifting is really sloppy - sometimes I get two gears for the price of one shift. It phantom-shifts when 1. it's under a load or 2. it's a bumpy road.

Does anybody have a fix for this? I'm sure it must be a simple adjustment, but I've fiddled with the shift-barrel thing (on the cable next to the front fork) and it doesn't seem to make a difference. Could the back cog be mis-aligned? Or is there another adjustment I'm missing?

Another question - anybody have a really good book on road bike maintenance they can recommend?