I had a Sigma computer that was 1 step up from the BC 1609 (mine had HRM). I did find that occasionally, the cadence signal was finicky. Usually, just fiddling around with the position of the sensor did the trick for me. My chainstay that it was positioned on, is sort of an odd shape...so the sensor didn't sit very well on it which is why I assume I would frequently lose the signal.
Now that I'm using a Garmin Edge 500, I haven't had the need to fiddle with the sensor ever since I installed it...as the Garmin sensor fits a bit more securely on my chainstay.
2012 Seven Axiom SL - Specialized Ruby SL 155