Both your downstream power levels and SNR ratios are quite good. I don't have cable Internet so I can't speak to the number of channels for upstream--do they typically only use 1 channel? I mean, it seems like a problem at the node or wiring on the street rather than with your home wiring.
Do you notice the crappy upload speeds happening in any pattern? Like, at peak traffic hours (5P-Midnight, roughly)? Is it possible they just haven't provisioned you correctly for Ultimate?