Bora V6 overheating intermittently

Hi all,
firstly, I did a search and couldn’t find anything with the same info as what I’m about to ask, but please someone correct me if I’m wrong.
I’m sorting a few issues on my VW Bora 4mo so that my cousin can take the car with him when he moves interstate.
When he’s driving the car, it’ll overheat and pop the coolant overflow cap every now and again. He says he can’t figure a cennection between every time it happens. Sometimes it’s in traffic, sometimes it’s just rolling down the road to the shops.
The catch is, I’ve taken it off him a few times, and it will not do it for me. It will always sit at a comfortable temperature, and the old girl seems happy. (I did give it a bit of a belt around Yarra bend a few weeks ago for good measure, but nothing).
The cooling fans come on when they’re supposed to, the thermostat/housing/crack pipe was all changed less than 2,000 km ago, and I took my time over a few good weekends to make sure it was right.
My guess is a) the coolant to water mixture is way off, b) there’s an air lock and c) the pump has gone, or is on its way out.
None of these make sense as to how it’s overheating seemingly randomly as far as I’m aware.

Does anyone one have any ideas as I’d love to see the old girl stay reliable for years to come. TIA!

I'd be looking at the water pump.

The impellor may be occasionally free-spinning on the shaft, and not actually doing any pumping.

Any way to tell if it is or isn’t working properly without just replacing it?

Have a look for black sediment in the expansion tank.

Sometimes, this is a sign that the impeller has failed, as this is the ground-up bits of plastic.

Also, get it up to temperature and watch for flow at the return hose on the expansion tank. This is the smaller hose at the top. You should see coolant flowing at the inlet.

Coolant is definitely clean, no sign of black stuff.
I’ll take a look at the return hose flow this weekend and report back.