Try removing the speed / cadence sensor and ride just using GPS (change setting on computer to remove the sensor), see if it is accurate that way. By doing this you will be able to at least eliminate the 500 as the problem. If everything works fine it may be time to contact Garmin about warrantying the sensor. Just so you know, they are great about handling problems.

Electronics, especially those with batteries, are more likely to fail after they've been sitting completely idle for a month or more. I'd say you've exhausted almost all of the troubleshooting steps you have short of running it just on GPS...probably time to stop frustrating yourself trying to fix it and get in touch with Garmin.

Agree with others here that it may be time to give the fine folks at Garmin a call as they are usually very helpful and cooperative. However, there is one more thing you can look at as I had similar issues with my 500 that ultimately turned out to be a rusted cadence sensor on the crankarm. The magnet inside the plastic housing was literally rusting apart... crumbling. Replaced it with a new one and made sure the alignments on both magnets were perfect and that seemsed to do the trick. Note that I also replaced the battery on the unit as well as several of the other suggested fixes here as well. Was discussing the issue with one of the riders in our group who had an issue with his 500 as well... he replaced the crank arm magnet with one of the "rare earth" button magnets that drop into the hole at the end of the crank arm where you tighten the pedal. Realigned the GSC-10 on the chain stay so it lined up with the new magnet; realignd the spoke magnet and arm and hasn't had a problem since. I decided to drop the $5 on eBay and try the same thing... Now all three of my bikes use those magnets instead of the stock Garmin magnet that zip ties to the crank arm. Works like a charm and no ugly zip tied magnet on my crank arm...

The reed sensor in speed arm of the Garmin GSC-10 is prone to failure either due to shock or water ingress. I have had two units fail in the exact same way in the past year. Failure will be intermittent at first but then absolute: auto-pause will come on randomly then eventually the green led will not light at all when the speed magnet passes. Get Garmin to replace it under warranty, or buy a new non-Garmin sensor. I like the Motorola or Bontrager Interchange ones. They appear to be a more sturdy design and can be easily swapped between bikes because they don't need cable ties to attach. The Wahoo/Timex type sensor is another alternative which is particularly good for curved chainstays because the speed and cadence sensors can be positioned independently.

Thanks. I will take the sensor off today for my ride. My situation sure does sound similar to yours,bones. I will look at other sensors.

I have the 2 part Wahoo and it is nice. I just ordered the Wahoo Bluetooth SC and it's the same design as the Bontrager/Motorola ones but Bluetooth. Nice without zipties.

Just got back from ride not using the sensor. Data is very close to that when the sensor seemed to be working. Speed still fluctuated a lot but it did not die at the end of the ride and distance was accurate as was average speed data. definitely appears to be a sensor problem. Thanks for all the help in figuring this out.

Glad you were able to get it figured out. Hope everything works out with warranty for you.