Change primary and secondary filters, likely the secondary is causing the issue.
Its possible to check lift pump pressure, though I'm not clear as to exactly how.
If you suspect its a line, run a line straight from the lift pump to a 5 gal can and see how it runs, just remember the return line is going back to the main tank, and will overflow until corrected.
if your filters are all sorts of full of shmoo get some biocide to kill the fungus what grows in diesel, from the sounds of it she sat for some time so either there is a bunch of water in the tank, or water and fungus.
Also Tank should have a drain on it to let any water out.