Wild speed jump readings with Garmin speed sensor

After two years of use, today was the first time I experienced wild jumps in my speed readout, i.e. jumps of up to 10 km/h up and down within seconds of each other. Am using a Garmin speed sensor on my front axle, with 50% battery charge according to the bar graph. I read all the connected posts and am wondering if there's a conflict with the GPS signal, which is active. However, can't figure out how to prioritize the speed sensor, as the GPS sensor is not listed in the sensors window. 

Any suggestions?