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It sounds to me as if your speed sensor were being intermittent... either only some of the time communicating its sensing of revolutions of the wheel... or only some of the time sensing the wheel turning.
When it is sensing/transmitting, the speed registered on the Edge 800 goes up... when it is cut off again, the speed registered goes down.
This would be more likely, if you have observed the speed drop toward 0, then going up not much farther than the actual speed. If it has gone up to obviously above the real speed, ever, then that is a different sort of problem altogether.
So I advise you to adjust and inspect and re-mount your speed sensor. Change its batteries, and clean out the battery compartment in it with cotton swabs moistened in rubbing alcohol.
Then try it again.
If it still works only intermittently, then the speed sensor is no doubt defective, simply, and will need replacing.
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