Did you get NAT-Type: open by open your ports (Steam & MW3 ports)?
Also this issue sounds like the typical IW5.0 (MW3 game engine) “problem”.
Many video setup parameters are put into the config.cfg by the game automatically and you can not change them via game menu settings! Optimizing these parameters in the config.cfg is allowed by steam/Activision:
>>>Call of Duty: Modern Warfare 2 & 3
VAC is enabled on all multiplayer servers, public and private. Any modifications to the game other than editing config files can result in a VAC ban. Examples of such modifications include but are not limited to console enablers, VAC disablers, and server hosting mods.<<<
…but it also needs some kind of basic knowledge of PC hardware / common graphic knowledge, so I would suggest you to get one of these >MW3 Config tools< which are available in the www.
These links will get you to steam discussions (with links to one of these tools) about this issue: