Lots of reading here about 'speedometer problems'. http://www.google.com/cse?cx=partner...ems&gsc.page=1
I never had much luck with the wireless type and I do about the same things as mapbike does. The odd thing is I had one bike that never had a problem with a wireless speedo but every other one had the same symptoms Scol, the OP is having. My wired speedometers all seem to work well.