I had the same issue. AFAIK it appears to be related to DX11 Tesselation.
The reason could reside either in the implementation details of CryEngine, or how MWO uses it.. it's impossible to try to guess any further
You can either:
- Revert the game back to DX9, or
- Set "Particles" to LOW
I have currently the game running on 1920x1080, DX11 enabled and:
- Effects: Very High
- Object Detail: Very High
- Particles: Low
- Post Processing: High
- Shading:Very High
- Shadows: Very High
- Texturng: Very High
- Environment: Very High
- AA: Off
--> Runs within constant 30 - 50 FPS on GTX 470, Core i7 860, 8GB Ram.
PPC doesn't look as nice as it used to, and the missiles have no smoke trail, but at least I can now aim while heating.