I've noticed fire often (not always) has visible "contours", I have that problem especially with fire spells.
Check these screenshots to understand:
http://cloud-2.steampowered.com/ugc/3281180233995747900/147FFCA229979EEEC72D12A7C17CDD6C7E5E9CE4/
http://cloud-4.steampowered.com/ugc/3281180234039090887/4F95C47823C2A32D6D67F9644B76002EAD1C1DAC/
My game is heavily modded but that shouldn't cause any issues with graphics.