2011 Mitsubishi L200 Warrior

So my L200 has been acting strangely.

When driving normally it never overheats but i had to collect a jets the other day for someone and it overheated. It seems to overheat whenever towing the slightest weight. Its will start to get hot but not in red, I pull over, release the radiator cap and coolant will fire up, top up with warm coolant and it will be fine for another 30 minuets or so.

On idling with the radiator cap off it has regular bubbles coming up but not foam. No contamination in the oil or cap.

So I have

Fitted a new thermostat no change
Pressure tested the radiator. When the engine is cold no loss of pressure. When the thermostat opens there is an increase in pressure with the engine running. When its hot with the engine off there is a slight drop. No visible leaks.
Tested the viscous coupling fan and all works fine when hot
Sniff tested the radiator with the engine hot and the test comes back negative

Any ideas..cracked head? But with the negative sniff test?

TIA

Jet Ski

And i have swapped the radiator and cap out for a spare i had

When my k74 overheated when towing or going uphill (. Basically under extra stress) it was a blocked radiator. A good flush and refill , making sure no airlock.solved it