Mazda 929 overheating

Sounds like the radiator may not be as effiecient as it once was. How bad was the fluid when you flushed it? There is always the water pump, but theres not much on those to go wrong. Just and impeller and a seal.
Try this test. When it starts to over heat, select full hot on your temperature thermostat. If the car runs normal temp, then just the added cooling of those fins in the heater core are all you are lacking for handling the heat your engine is building.
It may not be a bad idea to removed the water pump and see if there are any problems. Even have a new one to compare. If it looks bad, replace it. If it looks identical to the new one, then it probably isnt the problem. If it is not the radiator, then you must have a restriction of flow somewhere in the system. Might be the head gasket orifices, or just about anything.
Make sure the ducting and air baffles around the radiator and condesor have a good seal. You want all of the air flow to be directed thru the radiator.
This fan you installed in place of the clutch. I would say there is a possiblitiy that it might not be as effiicent as the original, but by removing it, does the water pump still turn?

If you do, get one of those flush'N fill kits from prestone and hook up the fittings that allow you to connect a water hose and back flush your radiator. Flushing it backwards helps try and free anything that might have clogged it forwards.
I have to say, very few flushes have resulted in great success, but its worth a try.