Getting double cadence reading on Echo F2 cycle computer

[beetle]

Since my LBS fitted my wired cycle computer I have been getting cadence readings that are double my actual cadence (ie. doing 80 and reading 160). I am doing all y session indoor at the moment. Has anyone else experienced this with their cycle computer before? Should I check the sensor mounting? I was going to call the LBS about it today but jumped on for my session and magically the reading was fine :confused:. Ideas?

[DebW]

Maybe you have a magnet in your shoe or somewhere that it's picking up from.