Thought I knew a little about engines, But!

Ok I need you G wagon experts to help on this one.

463 91 3.0 petrol 114,000 miles. owned 6 months and still trying to identify the problem.

 Symptoms.Engine overheated when hot day and travelling slowly. evidence by temp guage and expantion tank blowing.

Remedy tried, new radiator cap, new thermostat, new recored radiator.
Some improvement but still overheated on slow uphill journey or standing traffic on hot day.
Suspected head gasket but tests by coloured indicator fail to confirm.
Now 2500 miles later, some small loss of water, oil pressure good and no contamination of oil. Oil consumtion from max to min on stick in 2500 miles. small oil leaks around head/plugs.
Yesterday, drove at 70 mph for 10 miles and stopped. restarted after seven hours no problem. (some oil under car but may have come from stone guard,oil that was blown onto engine due to speed.) as engine came up to temperature it just went straight to maximum, I noticed imediateley as I was watching it because as I started the rad low level blinked.
I stopped and checked engine, it was not hot and I was able to remove the rad cap. water level slightly low. I continued home 10 miles with the temp guage going from 60 to max frequently. I controlled this by coasting.
Today I drove it a short distance with no visable ill effect except the temperature went up to 90 as it warmed.
So back to square one, a definate problem but what? can anyone come up with a possible cause?
Thanks in antiisipation,