I'd also like to know the results of these radiators. I have a 2000 xj that has a new Spectra radiator, new dealer thermostat, new dealer water pump, new dealer fan clutch, new dealer radiator cap. I have flushed the system 2x even though it was clean when I drained it. No rust, scale, or anything behind the water pump or thermostat housing. I have personally tested it 3x for exhaust gases in cooling system, pressure tested cooling system with no bleed down. It gets hot at idle in traffic or low speeds to the point the aux fan comes on at 220 via my scanner. At this point I can only run A/C on the highway. The factory clutch fan just doesn't move enough air at lower rpm's/idle. It does kick in at higher rpm's when hot as you can hear the roar of the fan until it cools a bit then slows down.
Now I have to assume you have something else going on.
'00 XJ with CA Emissions. Bought 7 years ago kinda running and not really driveable.
Put in a new Napa, rebranded Spectra, radiator with the cheap Napa thermostat and radiator cap.
Radiator was one of the things that needed fixing right away.
Runs fine. Temp always where I believe it should be.
I replaced the fan clutch at some point later not cause I was having problems but because I came across this video of what a properly operating fan clutch should do when the engine is hot and you shut it down.
Mine just kept spinning which indicated a bad fan clutch.
Napa XJ one which is just a rebranded Hayden.
Couple years ago replaced the water pump cause either coolant was dribbling out of the weep hole or the gasket was bad.
Napa one which is a rebranded Gates.
At that time I also replaced the electric fan not cause there was anything wrong with it but one of the tabs was broken and it annoyed me.
Used a $45 Ebay special there.
Going to assume you are running the fan shroud around the mechanical fan.