Garmin SpeedSensor 2 giving inconsistent speed

I have a speedsensor and an edge 1040 solar that has been great.

recently I noticed that the speed on my edge is randomly dropping to nearly half the speed I would expect. I would be risking 23 mph and then it would cut in half to 12 or so speed for a few seconds and going back to 23 mph. 

I thought it’s a signal problem, but it doesn’t look like it since there isn’t even cell signal where I ride.

i have tried changing the battery without any change.