I also went from the BlueSC to the RPM speed and cadence sensors. Have had them for just over a year now and haven't had the issue you are having. Every once in a great while, I may get a speed spike from the sensor but it usually spikes to the 30-60 mph range. It happens so infrequent that I personally don't worry about it. Not had that issue with cadence sensor yet. Have you contacted Wahoo support? They are really good about helping and very good about replacing defective products in a very timely manor.
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2017 Colnago C-RS
2012 Colnago Ace
2010 Giant Cypress