Thank you for getting back to me quickly. 

1) Yes I am essentially solving a diffusion equation in 3D, no convection with the UDS diffusivity defined as 1 to create a poisson solver defined here- https://www.afs.enea.it/project/neptunius/docs/fluent/html/ug/node1366.htm

2) Im using a structured grid with 4.5million cells.

3) I've ran both 0 flux and Specified as Static Pressure on the boundaries. 

4) I've done both SIMPLE and Coupled cases depending on compressiblity effects

The general behaviour would be for the first 10-15 iterations to show convergence (around 5e-5 to 1e-7) then a continued uptick until the solution diverges