wrong speed durin activity

What GPS settings are you using?  Is this the first time it has done this , or does it happen often? If you overlay with something like heart rate , does the heart rate dip at the ams points indicating likely slowing down?  Have you got 3D Distance and 3D Speed turned off?

Check the topics about the gps problem since 2.37 gps software. 

Look at your course map if it's also messed up.

Ok, I'll do it. 

The map is not messed up when the speed is going to zero, my activity is kayaking but the way looks as usual.

What GPS settings are you using? GPS+GLONASS+GALILEO

Is this the first time it has done this , or does it happen often? It hapens often

If you overlay with something like heart rate , does the heart rate dip at the ams points indicating likely slowing down? 

I'm using Garmin HRM band  but can't apreciate nothing wrong about this

Have you got 3D Distance and 3D Speed turned off? I don't know how to do it

Try disabling HRM Pro pace and distance and auto calibration. Or change it to bluetooth. 

It once solved for me with the Fenix 7 after these new functions were implemented from the HRM Pro plus.

The 3d speed etc is under each activity setting . Make sure they are both off.