For my tuppence worth, having had a vehicle that used to overheat even with a brand new rad, thermostat, hoses and water pump in it, there have been many good recommendations on here.
Viscous fans tend to fail seized, ie locked to their shaft, so would normally provide plenty of cooling and would be running at high vehicle speeds when the ram airflow through the rad has cooled their hub and should have caused them to disengage. However, just because the normal mode of failure is to seize, that desn't preclude a free-spining hot viscous hub, so it's worth checking that the hub stiffense correctly when hot.
Cleaning out the entire cooling system is very important, not only the rad itself. I used a warm caustic soda solution to remove deposits from my block and rad, and i think it may be that which ended the overheating problem.
Electric fans could help, but they shouldn't be necessary if the original viscous fan is fitted - that fan has enormous capacity.
R LeFebvre's (Rob?) comment about the fan shroud is very important. With the fan removed, ram airflow on the motorway may be improved, but the fan is made very inefficient and will struggle to pull air through th rad at low vehicle speed, the airflow taking the course of least resistance from around the rad before passing through the fan. If the shroud has been removed, refitting it could make a huge difference.
Engine breathing: Exhausts don't just affect power and torque - they affect engine temperatures too. At the same time as I caustic flushed my engine (2.5nad), I also fitted the correct size 2" exhaust, replacing the earlier SIII exhaust. It's possible that this had the main cooling effect, rather than the flushing - having been done simultaneously, I'll never know. So, a dented, split or blocked exhaust could be the cause.
Coolant composition is also very important, with plain water or diluted solutions coping less well than those with water wetters or antifreeze.
I hope you get to the bottom of it, whatever the cause is.