Constant FPS Spiking

Recently the game has been constantly dropping frames. Limited the game to 60 and it tends to run at the top, then suddenly it'll spike down to sub 20, often to around 9 and sit there for a few seconds before going back up. I've tried disabling DPI Scaling and reducing my graphics nothing has worked. PC is fairly old, but doesn't struggle with any other games, also the spiking isn't just during fights, it can occur during movement and even just sitting afk.

Any help/advice/ideas would be greatly appreciated.