I don't mind accepting blame for a faulty breadboard setup "on my end", if that is the case. I have no delusions that that will not happen, and even happen a good number of times.
But I've now gone through a whole list of possible faults and recommended remedies, and nothing seems to have done the trick. I have also tried to build an Atmega1284P-PU circuit on a breadboard with the absolute minimum essentials that are needed to upload sketches at all, and that too has not worked.
If it's still somehow my fault, then I have no problem with that. But it kind of seems like it isn't so.
@CrossRoads: I will look at your suggestions in more detail as soon as I get home.
- carguy