I have Sapphire pulse rx 5700 xt.. I've just bought it today and made a clean install with drivers.. My old rx 480 was running on full gpu clock speed but this one is fluctating.. and thats causes my games to stutter.. I supposed to take like 110-120 fps on gta 5 ( 1080 p ultra) but because of this fluctating, i cant take it. 30 fps, 40 fps is highest in my game. Sometimes it hits 60 fps but instantly go down..
My core clock needs to work at 1800+ mhz and this fluctating made them like 1000-900- 1200 mhz..
Try to create a separate profile for GTA 5 in the driver settings. Experiment with the shader cache option (on or off).
I do not play GTA 5, however, in the game Watch Dogs 2, disabling this option allowed me to get rid of serious friezes. And in the Black Mesa game, on the contrary, inclusion helps.
However, this did not remove the small periodic FPS drops. But playing became much more comfortable.
I have the sapphire Ntiro Rx 5700 XT and i have the same exact issues with the LOW LOW FPS on 2k ultra. I did how ever find the issue, the problem is with the shader quality if you have that on ultra you will get roughly 20-26 FPS during UGINE Superposition Benchmark. The second you take the shader quality from ultra to high i instantly was running 70+ FPS which still isnt great for 2k on this card. I believe it may be a step in the right direction. (hopefully)