ross replied:

I would get the mechanic to do a compression test and if it fails dont spend anymore money.
The old 2H AT 500000klms is probably just about dead. They do go longer but only with impeccable servicing and ownership.
If it somehow has some good compression figures you will be looking at the fuel pump and injectors.
Leaks in the fuel system could be letting air in ,check around the primer pump particuarly

Is it making lots of exhaust smoke,if so what colour?
What does it start like ,does it run rough like its not firing on all 6 cyl?

Member - Tour Boy replied:

Hi Royce,
My brother in law has the same symptoms in his lux after getting a gut full of bio from the nsw central coast. His truck probably had crap in the fuel system from a heap of years and kays (ex railways) After he used the tank, he changed the filters again, went to a major brand servo and presto god as new again.
My old 2H with an axt turbo in my stretched cruiser similar to yours had 440,000km and still going strong when I sold it to another operator even after 9 years on the Canning with a trailer. The motor finally was replaced with a newie 5 years later probably 650,000km
Regards
Tour Boy