Just look there
http://www.gps-sport.net/forums/thread/1333-0/Zephyr-HxM-heartrate-drops
There is the same problem with this "lock" phenomenom as you described!
After reading through the forums about this heart monitor I decided to get one. I am a complete beginner in running but like all the data to analyse after a run to see how much fitter I am getting, todays data showed until about 80% of the way through the run I had a heart rate of 59 and then it shot up to 189 for the last .5 mile.
I am having problems with it how tight it supposed to be as mine keeps slidding down and how much water I should put ont the belt to help with the signal.
Why does it report after I have started (about 3 mins into the run) heartrate monitor bad signal x2 and then say its diconnected?
Is it more trouble that its worth? Any help appreciated.