Is there any pattern to when/where it happens? And how recently?
If there's no commonality in-game (e.g. the area you're in / the number of other players in the area etc), then I'd be inclined to suspect there's another process running on your PC competing for the resources the game needs - are you using integrated graphics, or are you using dedicated hardware?
If you open the developer console (hold alt and press `), and enter the "renderer" command, then what does it say for the top few lines (before "Capabilities:"?
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