Agreed. When I had my 305, when signal was lost, the cadence sensor would give me all kinds of wacky readings.

I've observed the same thing as the OP. In Garmin connect I would get a higher average speed which wouldn't show up on the ride data. I have a feeling that it's simply a stray data point. Usually when I plug the data into Ascent, it's gone.

After my third 305 died, I got a 500 and must say it's a significant improvement over the 305. It's faster, more accurate, and doesn't lose signal like the 305 did. I can turn it on indoors and ride under large canopies of trees and never lose signal in places the 305 would drop out frequently.