1996 Other Isuzu Models Stalling.

I just purchased a used 1996 Isuzu Trooper 3.1 turbo diesel, rhd (right hand drive) vehicle. After cleaning and changing all the fluids and filters noticed stalling issues. I have pretty much sorted out idle and stalling problems by bleeding the lines and cleaning or changes fuel lines as well as removing and cleaning tank. (Possiblely poor diesel fuel source put in after I changed all filters) now I am working on second tank of fuel. Over all the truck runs good, lots of power with turbo boosting engine. Starting sometimes will need to crank over for a few seconds before it will start and then run. Though on other occations it will fire up and start almost immediately.

The problem is while on an incline and idling, after 20-40 seconds sometimes longer, it will stall. (As engine revs start to go down, close to stalling, you can rev engine to maintain running, but idle speed will need to exceed 1,500rpms) engine will restart with some cranking but will not run well until vehicle is level. While engine is still warming up it will stall quicker on same incline. While I have not tested all possible situations, once the vehicle has been driven around and warmed up and or has been driven for a period of time, I have driven up steep inclines for a few minutes with no issues. (Driving up inclines at slow speeds, 1,200-1,500 rpm.) While the truck is on a decline will idle and run indefinately. When I say incline/decline, generally steep, 35-40 degrees or more.
I live in Solomon Islands, which has such steep grades and more. Fuel source is questionable, but that is all that there is. We can get fuel aditives for diesels from Australia. Climate is hot with a few days hotter. High humidity. Stalling condition unknown prior to purchase. Any suggestions on where to look to elminate the stalling?