My rain-bike/commuter is a 2006 R1000 (caad8) with a truvative gxp double and ultegra 9-speed. When I pedal, the slightest variation in my stroke causes "dead spots". I pedal and then there seems to be a "slackness" (?) in the stroke that is taken up as the chain reengages the drive train. I wish there was a more succinct way to put it. I don't remember it being an issue last year. I did have issues with the left crank arm getting loose last spring (a little lock-tight remedied it) so I wondering if the problem might be there or in the freewheel hub (crappy but relatively bomb-proof wh-r500). I do all the repairs on my bikes but I was hoping to get pointed in the right direction instead of doing a full tear-down/rebuild of my everyday commuter.
On a positive note, it have really helped me smooth out my stroke since it drives me crazy when it goes slack.
My other roadbike (2010 supersix with 6700) does not seem to have this problem.