This post has been marked by the post author as the answer.
Did you happen to adjust the terrain LOD sliders in the matrices panel? Setting them to low has caused this issue before.
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.
