The slider in the graphics options needs to be above zero
The sliders in the post effects options control it more. Try more decay. less quality, and a medium length.
See if this works. I have noticed that sometimes you need to set them, save and reload the map.
Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz, 2400 Mhz, 4 Core(s), 4 Logical Processor(s), 8gb RAM, Nvidia gtx660, Windows 7 Pro 64bit, Screen resolution 1680 x 1050.
