Radiator blockage could happen due to foriegn substance like stop leak, or non-honda coolant.
I assume no sign of oil in the coolant? That would look like a chocolate milk shake and would definitely degrade cooling.
Also radiators don't last forever, they wear from flow and corrosion, and the new ones will lose their seals to the end tanks. Corrosion could also reduce flow and heat transfer, although I don't think you'd get much corrosion build up with honda coolant. If the mileage is anywhere near 200K I'd just replace the radiator.
I guess it's remotely possible that water pump is damaged or worn, but that usually results in leaks or a burning belt if it jams. You'd probably have to remove it to inspect the impeller, and at that point just replace it.
You know ... That can be a possibility; the block may be cracked. I was a little more optimistic and didn't consider that at all ... Block tester? Hopefully no bubbles or liquid color change. But aye ... Engines are going for 500 on ebay at the end of the day.