Try running through Compressor — you’ll get a lot more control over everything, including gamma, color, etc.
Export a reference file: a non-self contained MOV using current settings. Pull that into Compressor. Pick a MP4 with the bit rate you like. Go through and adjust frame size, final bit rate, color and audio controls. Submit it to a cluster that you’ve setup in Qmaster for better speed in rendering. You can also set it up to send it to the web for you, etc.
Good luck.
Non-linear: all the time and nothing but.