Overheating problems/NOS quality question
 

I have an '04 Freelander with ~125k miles that has been overheating for a few months now. So far I have replaced the radiator, thermostat, and water pump.
I have done all of the work myself so it hasn't been too expensive, but at this point I just don't know what else to do.
It will, even sitting at idle, slowly creep it's way up past 220 degrees F over the course of about 20 minutes.
The only unresolved issue I have found is that when I pulled it apart to do the water pump, I could spin the pulley of the old water pump with my hand against the timing belt.
The tensioner on the timing belt is a NOS OE Rover part made back in the early 2000's based on the logo on the box that it came it. It's been in there for about 10k miles so it should have plenty of life left in it, but I don't know if being 15+ years old would be long enough for the o-rings to break down and start leaking a little bit.
When I put the new water pump in, I tensioned the belt with an allen wrench and it was tight enough that the water pump pulley couldn't slip. But I'm wondering if over time it is not holding quite tight enough for the water pump to work properly.
Has anybody else seen anything like this before I go back in there again with a new belt and tensioner?

I heard LR used glue to hold the engine together ..