FDC is "Forward Data Channel" (from the headend to your STB) and RDC is "Reverse Data Channel" (from your STB back to the headend). Check this post for details on what these all mean/do: »Re: [DVR] cisco rng200n dvr
As far as the differences in reception for different channels close in frequency, a coax cabling/connections issue can cause problems at a certain frequency or range of frequencies. For digital channels, if the QAM carrier used to carry a particular channel happens to fall at that frequency/frequencies, then there could be issues for that channel [and any other channels carried on that QAM carrier(s)] while other channels, carried on carriers outside the problem frequency(ies) may be OK.
Replacing the splitters you can reach and that line for the upstairs TVs may resolve many of your issues.