fuel

There is more fuel pressure when the tank is full.  Look for partial blockage, screens, filters etc. 

 Welcome Dean ,

Totally agree.

You may have to remove the sediment bowl assembly to check the in-tank strainer.

There is also a mini-strainer in the lift pump suction banjo bolt.

This symptom also suggests the onset of diesel bug contamination. See if there is a dark brown or black sludge anywhere in the fuel system.