Want to bump this thread, as I have the same issue as MisterMagoo.
I'm on latest firmware (F-GGB-2O-1407041 from July 24th 2014).
Only using cadence and HR sensors (both the original that came with the GB-580).
I only have problem with the HR sensor. It spikes to 240 after maybe 25-30 minutes of use, and then stays at 240 for the rest of the workout. Sometimes it goes back to normal, but not for long.
I have changed the battery in the sensor, and re-paired it with the GB580 (turned HR off, then scanned for it again).
I imported the workout in SportTracks, and this is the graph of the HR. I highly doubt that I ever was above 190bpm. Also plotted cadence, so that you can see that this sensor is always working.
As the cadence sensor is working, I guess that it is the HR sensor that is messing up.
I also have the .gpx file (or any other file-type of the workout) if this will help.