When stopped 1040 keeps moving drifting? How to stop this from happening?

it is recommended to pause your activity because you are experiencing what is called "GPS Drift".

"GPS will continue to record while you are stopped unless you pause your activity. Distance will accumulate while you are stopped because several points will be recorded around your position due to GPS drift."

Please see this FAQ: How to Improve GPS Distance, Speed and Pace Data With Garmin Devices 

I don’t ever remembering this happening before. I need all the data, stoped time, avg moving speed, total elapsed time and over avg speed. Most of theses events last for 30 to 60+ hours. 

If I was to add a speed sensor would that fix my problem? Or would it be less accurate being I’m most of road MTB? Or does the gps and speed sensor work together?

Jwharton said:

Or does the gps and speed sensor work together?

Yes and no:
speedsensor overrules GPS. If a speedsensor is connected, then - see @L.Rouge comment!

But: Garmin speedsensor 2 disconnects (goes to sleep) after 6 minutes of standing still, and then GPS speed measurement kicks in - with your problem. When moving again the speedsensor will connect again automatically. (This is my experience at least…)

mcinner1 said:

Garmin speedsensor 2 disconnects (goes to sleep) after 6 minutes of standing still, and then GPS speed measurement kicks in - with your problem

That would be very annoying, I have a older Garmin speed sensor, looks like I should avoid replacing it, the behaviour you describe sounds to me to be a design fault, my older one only needs a battery change each year or so, disconnecting to presumably save battery is therefore pointless.

L.Rouge said:

looks like I should avoid replacing it

I bought the speedsensor 2 to replace my gen1 sensor, which was still working.
After I had found out this 6 minute limit of pause-life a swapped back to the old gen1 sensor - till today.
(gen2 sensor is still unused…)

In My experience Speed Sensor does not overrule GPS as along as some GPS is available.

I have to cross a river for half of my riding area. It’s a big one and on the bridge pillars technology to guide ships is installed.

This irritates the Edge when crossing the bridge, if I go „downhill“, it jumps between 26km/h and up to 40. The higher number being usually the real one. It happens with and without a speed sensor and is also recorded. 

BeneGi said:

In My experience Speed Sensor does not overrule GPS as along as some GPS is available.

The speed sensor definitely overruls GPS speed measurement when the sensor is connected.
But the speed sensor uses the earth's magnetic field to detect rotation, and this is disturbed by massive iron structures or magnetic waves.

To check: connect the speed sensor to the Edge, make sure that you also have good GPS reception. Now turn the wheel to which the speed sensor is attached while stationary.
If you can read a speed, it must come from the speed sensor!

The activity file also identifies the source it uses for information like speed throughout the activity. So this can be checked post ride.

The above test is a simple demonstration  of the source the Edge is using.

Depending on the brand of speed sensor it maybe affected by environmental factors. Magnetization of the drive chain can impact the speed reading on a rear wheel mounted sensor that is using a magnetometer. 

If you are using a speed sensor I would suggest checking the setting for the wheel circumference is correct. The Edge will automatically compute a size and will continual monitor that value and adjust it if set to automatic. Personally I always manually set the size so that it is a constant.

As mentioned above if the Edge loses communication with the speed senor then it will revert to using GPS.