I noticed that my cooling fan can cause enough interference with my stages to interrupt the Ant+ signal . You see random slow down in speed due to lost data, normally I ride with the fan directly facing me but have switched it to the side to reduce interference with my suunto dongle which is in the pc in front of my bike with the fan between me and the PC previously.
There is a thread here on ant+ connectivity issues and most of us have got USB extension cables to solve problems like this. But also limit other wireless devices on a similar frequency range to help or if you can.
I took your advice and used a USB extension to place the ant+ dongle under the bike and the system is working much better