Screen flicker on new GPU

yeah that's bad GPU or memory... still warranty? -> ask for a new one

EDIT: in that vid I cannot see anything

Hmmm, my first reaction would be the PSU as well. Do you use a surge protector? Try plugging directly into the wall and see if the flicker still happens

Otherwise, like 1337Ox said, may be time to utilize that warranty

Are you using analogue VGA connection, or a digital one (DVI/HDMI/DP)?

its the gfx card, several friends have had this on the r7 260-265 series, and tbh the replacement will most likely do the same, i know mine did, bad or faulty design.

This happens when you use adapters, try directly plugging in via DVI, if you do not have a monitor that supports it, try it on someone else's PC who does. Also you can try getting a better VGA cable, although they are hard to come by these days.