Cyclometer interference from car detection loops?
The car induction loop seems to be the most likely culprit. Their operation rely on an induction coil being buried under the street in front of the lights [1]. When it detects a car, or for that matter your metal bike, as a result its magnetic field is increased. This magnetic field is used for operation of the lights, but its conceivable that the field will affect the operation of your wireless comp. It's questionable if it would affect a wired computer, as it would be shielded with the connecting cable.

When passing through the door in my local bike shop with my bike while having my cyclocomputer fitted I also see the same affect. This time its down to the anti-theft magnetic detection loops fitted at the door. [2]

I suppose the solution is to fit a wired comp instead. Or else make sure you stay well back from/in front of the lights.

[1] http://auto.howstuffworks.com/car-driving-safety/safety-regulatory-devices/question234.htm

[2] http://en.wikipedia.org/wiki/Electronic_article_surveillance#Magnetic_systems