Erratic “EKF GPS Fusion timeout”

I had two really good test flights today with my Z-84 Flying Wing, but in a couple of occasions I got “EKF GPS Fusion timeout” messages forcing me to switch to stabilize mode. Does anyone know what triggers the “EKF GPS Fusion timeout” message? Here the logs from one of the flights:

You are having some problems with your GPS module or the wiring to it. You are seeing a lot of drop outs of GPS data. Not sure why exactly, but you should check your wiring and the GPS unit itself for problems.