I had the same problem as you, tried everything, samer problem, I went to the canon 20D, Problem Solve. I can shoot at ISO 1600 and be equivalent to ISO 800 etc. and more frames perseond
Looking at noise charts and comparing the noise of the D70 and the 20D I find that the D70 has less noise at 800 than the 20D and a a bit more noise than the 20D at ISO 1600.
In neither instance is this signifigant. Both are fine camera's, and both will giove good or poor results depending on the photographer.