Assuming you have the speed sensor wired correctly, is any part (or all?) of your ride under/near hi-tension power lines? Much of my riding is on the W&OD trail in Northern VA (major power lines from close-in to Washington DC out to Leesburg VA). My Polar 720 (digitized HRM with wireless speed and cadence) consistently flips out on one or all 3 measurements at various points along the trail. Fortunately (ghack!) I can edit out the data when I download to my PC. FWIW, my old analog HRM (Polar 150) was even more succeptible to power line interference. I don't know 1st-hand if the wireless Cateye or Polar cyclecomputers are as sensitive to interference. [But if anyone wants to send me some to test, I'll write up a full report ]. My old Cateye Astrale 8 (wired) was immune to interference (of course), but didn't provide all the bells and whistles.