Garmin Forerunner 255 time is always 2 or 3 minutes fast and Altimeter shows thousands of meters of ascent during an activity

Hi there,

My Forerunner 255 is always 2 or 3 minutes fast. Right now it's 18:40 in Ireland but my watch says it's 18:43. 

If I go outside and start an activity, the time changes to the correct time after it syncs with GPS. But then after the activity is finished, it goes fast again. If I go into Settings, Time, Time Sync and do a sync with my phone, the time stays fast, even though my phone time is correct

Also, for the last few weeks, when I do an activity, I get massive changes in Altitude. A recent 10k run around my town showed 3,400m ascent and nearly 2,000m decent. And a recent cycle showed over 11,400m ascent and 10,900m descent!

Has anyone seen these issues, and any ideas on how to fix them?

Thanks!