Dexta fuel problems

Yesterday I started it up and after driving about 50 meters she began to miss and died on the roadside. i pushed her back to the yard and began to do some investigation.I opened up the fuel filter bowl( suspecting dirty fuel)the filter was covered in brown gunk, I drained the tank and fitted a anew grossland filter. Iopened the fuel lines from filter to pump in addition to the nut in front of the injector pump in order to bleed the system . eventually the engine fired up. i ran it for 20 minutes with no apparent problems. i drove the tractor for 100 metres and just coming into a hill she began to miss and died again. I repeated the procedure outlined above and again the engine fired . I idled it for about a ahlf hour. QAgain when i went to move uphill she began to splutter and died onec again.
I reckon there could be several problems:
1. the system is gathering air somewhere
2 not enough fuel in the tank(currently about 1 gallon
3 .a very dirty fuel tank
I would be grateful for your ideas
Thanks
Henry J