Try a lower resolution and turn down (or off) some of the more GPU-intense effects, such as shadows... especially if you're experiencing framerate issues on the main menu.

Verify your game files through Steam, or even go completely nuclear: Uninstall Valheim, then delete the Valheim folder from your SteamLibrary folder to clear out any cruft, then reinstall.

Turn off your PC, open the case, remove and reseat your video card. Blow out the dust while you're in there (use "canned air", not your breath; blowing little blobs of spit on everything will cause more harm than good).

Uninstall your graphics drivers completely, then reinstall clean.

Check your temperatures and voltages; those can cause crashes to desktop, or even full-on blue screens. There is a plethora of monitoring software available for free online, if something isn't already installed from the motherboard or system manufacturer.