I thought of something else, reminded by Raymond's experience with the loose connector.

I took my bike in for something unrelated to the cyclometer, and when I got it back I noticed the mechanic had put some kind of waterproofing sealant that resembles putty, but is solid, over the semi-exposed connectors of the computer. I hadn't asked for this, but was glad to see it.

Could you have got some moisture inside your computer?