I had that problem when I used pixel sizes other than for example 512, so if I
used 500 I would see edges, if that helps stick with 128, 256,512,1024 etc..

defiant@zovera.com A_6 commercial 6.4.2 DirectX 9.0c p4 3.4 geforce 6800 ultra 256 mb 470 mhz forceware version 81.95