Did you try to reset the 305 unit? I have not had any trouble with either the 305 or 705 but I had a Suunto T6 that would not sync to the cadence unit until I temporarily turned of my wireless network.

From the Garmin site:

Q. How can I reset an Edge or Forerunner 205/305 unit that has locked up?
A. If your device has locked up during use, you can reset your unit by pressing the "Mode"(reset/lap) and "Power" buttons at the same time. In addition, please make sure that the device has the latest software version available. To update the software, please use the following link: