How narrow is the case?
I wonder if you just have too many sources/sinks for air and it's not keeping the flow clean... if there's too much mixing, it can be counter productive.

Since you have 2 exhausts at top near CPU and only 1 on video cards, air flow from the inlet at the front would want to go more towards CPU rather than the video cards... try turning off the case exhaust fan (or lower speed setting) at 120 mm exhaust fan to see if that helps... also keeping the areas where air flow occurs clean would help.