I'm using a RX 480 and it is good for playing games, but it does not have the power to run most games maxed out at 1080P, as neither does my Fury X. Some games can be run maxed out, but not all.
What temperatures is the GPU hitting? I can provide some recommended Wattman settings that should keep performance steady and temperatures in check.
Rise of the Tomb Raider is far too demanding to run at maximum settings. Using x4 SSAA you are effectively running the game at 4K resolution. Dial back your settings a little and things should stabilize.