I have Vega64 too, Sapphire, and exactly the same problem. It is MOST annoying, when you play racing games
Maybe something wiht refresh rate? or active sync? I switched from DP to HDMI and it is better, but still it is happening. But only once per the game session, never twice or more ! it is very strange.
I have this issue from very begginig.
Please help us !
OK I did these things, it helped me:
1) Turn freesync monitor and AMD card OFF
2) switch HDMI from 1.4 to 2.0
3) Turn both freesync ON
Now it is ok.