mocolotion

I ride with a Bontrager Node 2 computer connected to a Garmin GSC 10 speed and cadence sensor. On occasion I'll notice that the speed displayed on the screen is way above what the speed I'm actually moving, i.e. I'll glance down and see 74mph when I know I'm going closer to 30mph. Normally I would assume that the distance between the sensor and the spoke-mounted magnet was off so I wouldn't worry about it, however last night I noticed the speed readings were off my entire ride and this was after I stopped and made sure the magnet was passing within a few millimeters of the sensor.

Today I went out and picked up two new 3v batteries; one for the actual Node 2 and another for the Garmin unit. I swapped them both out, re-paired the sensors, and still appear to be getting ridiculously high speed readings. I had my bike upside down in 39x21 and was turning the cranks at a relatively slow pace and the screen was reading 42mph +/-.

Does anyone know what could be causing this? I'm hoping the unit didn't spontaneously kick the bucket on me.

Thanks!