Ciao ​@Karim mouden are you recording directly on your phone? It seems to be related with not correct gps data. Have you checked the map? Can you see something strange on the route such as straight lines? I see it in your print.. your speed near the first peak is linear and then strongly increase

Oui j enregistre depuis mon téléphone ça fait presque maintenant un mois que Strava calcule des données erronées 

Have you updated the app to make sure you have the most current version?  Also, how are you carrying your phone?  Is it in a bag or pocket where it might be getting a poor signal?  Have you tried deleting and reinstalling the app?

No, I've never updated the app. I put the phone in my pocket. I've never uninstalled the app.

I would recommend updating to the newest version of the app and see if that fixes the issue.  Also, having the phone in your pocket could be blocking some of the signal, but I would expect that to be more of an issue spread out throughout the activity and not just in one specific section like it shows in this file.  

J ai fait la mise à jour mais le problème persiste toujours