has the car been run on WVO ?
The problem is the dealer will not fix things. They just keep replacing stuff till the problem goes away.
The tank needs to come out & be cleaned. The best way is to steam it. I have cleaned tanks by taking them to a factory with a large boiler (cannery) & steaming the tank for a day or 2.
The fuel lines could be blown out with compressed air.
replace filters & keep fingers crossed.

It could be a case of bugs in the tank. Has he cut a filter open?

Hope this helps !

Good Luck

__________________
Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving