Cateye computer gone mad

[Redlight]

My cateye wireless computer has suddenly turned itself into a random number generator. One moment it tells me I'm doing 50 mph, the next that I'm doing 10mph or standing still.   The batteries in both the computer and the mount are new and nothing else has changed on the bike. 

Any suggestions?  The only thing I can think is that it's 6 years old and these things have a limited life.

[rower40]

Do you have LED lights nearby?  Particularly ones that flash, or dynamo ones (e.g. B&M IQ Cyo or Lumotec)?  These generate so much EM interference that wireless speedos stop working.

[Biggsy]

I've never heard of a cycle computer wearing out.

Have a fiddle with the sensor and magnet.  They might have moved a tad, even if you think not.