My BC 1200 turns itself on when I start to ride. The auto start feature is listed in the instructions and I think on the outside of the box too. You can use one of the smaller elastics on the fork to hold it better. Take the tape off of the glue on the back of the sensor to get it to stay still.

All six of my Sigma computers have worked OK for years. Some of them come off the shelf at the LBS with batteries that need replacing soon. That happens to other computers too.
The one with 8500 miles on it has never given me any trouble.