If the crashing has started since you downloaded the custom content (specifically markings/items packs) chances are that it's a problem with that. Flourite is a densely populated map and if numerous users are using the custom content at any particular time there it could just be that your computer isn't processing it well. I encountered issues before that sometimes cause my game to crash after getting that content.
The fix may be:
- to uninstall/reinstall the add ons to see if there is perhaps a broken file causing the crash. (Also pinpoint which add on is causing the issue)
- Change video settings in the options menu. Not just switching the rendering subsystem to OpenGL or Direct3D9 as you mentioned but also turning the View Distance to it's lowest setting, and possibly remove water reflection.
- Uninstall the content (if worse comes to worse)