Don't trust the jug, jug being the radiator reservoir jug. I've seen this problem a lot. Your car or truck seems to be running hot, but you have plenty of coolant in your radiator reservoir jug, so you think it's fine. But it's not and your engine gets ruined.
This is especially serious on your Isuzu npr, Isuzu nqr, Isuzu ftr and frrs, the UD1200, UD1300, UD1400, UD1800, UD2600 trucks because they are much more expensive than cars.