7 months is a long time for no driver updates, make sure your drivers are updated, if that doesn't help, switch the Graphics API back and forth between Vulcan and Direct x, 12 DX is the most stable and looks better imho.
But Vulcan seems to have the most stable memory usage, but that shouldn't matter if you have at least 50gb free on your main HDD/SSD drive.
It could also be the weird bug that lags your keyboard when you tab out and if it is all you need to do is run the benchmark test when you load the game, at the main menu, click story then settings at the bottom and hold x, run the test for 5 seconds and the bug'll be gone.
It's more than likely a driver update or a hardware issue.
ALSO verifying files through steam or rockstar launcher sometimes fixes FPS issues.