Forerunner 735xt

Do you mean the screen is flickering really fast?   Mine started doing this in the last couple of weeks and in the last couple of days the battery has started to drain within a couple of hours of fully charging. 

No, the watch is constantly buzzing/vibrating

Have you tried a master reset using 3 buttons

power device off 

hold watch with on/off close to left thumb, next hold the two furthest buttons ok and cancel with your index fingers of left and right hand, keep these pressed and turn watch on with left thumb, release thumb but keep fingers on other two buttons, when you hear first beep release left finger on second beep release right finger, this will reset everything so you will need to go thru setup and phone pairing again, see if this makes it behave

Thank you, I haven't tried this.  Could you confirm more about which buttons to press when - the forerunner 735xt has 4 buttons, two left and right.  The top left button is the complete "turn off" button. Thank you

Mine has 5 3 on left side 2 on right side thumb on on/off and fingers on 2 buttons on right side

thank you - it unfortunately doesn't work.

Contact Garmin Support via phone or chat. They'll walk you through some potential fixes.