well the reason the temp changed so fast is that the gauge in the DII's only has 4 positions. One for cold one for warming up, one for normal and one for overheating.
When it goes from normal to over heating its like click done! Then as it cools it drops from full temp back to normal in one click.
Now back to the condition.
The first thing I would be doing is replacing the thermostat (funky piece that it is) and flushing the system and replacing the coolant. Then make sure to fully bleed the system to make sure it dosent have any issues.
running with a fully loaded rack hard up a grade can heat up one of these trucks but it should still not over heat.
I got a bottle of Aloe after getting sun burned. It says "apply Liberally"... I'm not sure if this means other people pay for it and I use it, or I am to force other people to use it even if they dont want to because it's what I think is right for them?