I have an Avanti Discovery 8, which is a mid-range hybrid bike with Shimano in-hub gears. I ride about 20klms a day on bike paths so whilst it gets a work out, it never really gets much bad abuse. I've owned it about a year.
Yesterday I found that the chain would slip and catch occasionally when I was applying torque, like going up a hill. When I got home I checked all the links in the chain were flexing, moved the rear wheel back a few millimeters to tighten the chain, and tightened the crank (it was a little loose and wiggling a few mil) and re-oiled the chain. I also ensured the gears were aligned.
Next morning the bike felt fine until halfway to work then the slipping started again and increased as I rode. It got so bad that by the time I reached work I could no longer ride. Every stroke the chain slipped.
I can't figure out why this could be. It's not the gears, as there is no derailleur. Could it be the chain is actually stretched?