Overheating 1HD-T 80 series - your thoughts?

[Axis]

Hey guys, new here!

 

I've got an old 1992 1HD-T 80 series with about 400000 km's on the clock. We've put a fair bit of work into her recently including a recondition on the turbo as it was leaking oil badly. Mechanic had the head off as well as it was blowing a lot of smoke - this was all fixed and it was running like a dream for about 500kms...

Cruising along the highway - temp goes up into the red. Sat on the side of the road for a bit and it was clear that it had dumped most of the water from the radiator - apparently through the overflow. Ran it past a local mechanic where we were - he suggested thermostat and radiator cap as a starting point. Did these but no change: overheated twice more - both times going off in about 20 seconds from rock steady in the middle to right off the gauge. Took about 4-5L of water each time.

Limped home and back to the guy that did the work. Here's what we know:

1. Oil is clear - water is not going there
2. No oil in water either
3. Pressure test on radiator is a-ok
4. Ran it at idle for about 3 hours with pressure tester in place - no increase
5. Put a CO2 tester on radiator as well which was apparently clear
6. Running it around town is fine - but after a few days the reservoir will be full and the radiator will be down 1L or so
7. It's keeping pressure in the radiator overnight and not sucking back from the reservoir

I think that's everything! He was initially saying 'it's not the head' but he's now changed his tune and wants to take it off again next week - thankfully on his time (warranty) but still...

What are your thoughts?