Def 90 power loss problem

I have an intermitant power loss problem, I changed the fuel lift pump about 6 months ago. After the land was parked with the rear downhill the power suddenly disappeared, after a while it came back but it is now so bad I cannot drive it. There is sometimes a sound of fuel returning to the fuel tank when I have turned it of, can anyone give me some Ideas please I would be very grateful

after lots of investigating, changing the fule filter which seemed to help it turns out the lift pump is pumping diesel out of a hole in its side/bottom. difficult to see. I will change the lift pump tomorrow and investigate the hole. I am hoping this will sort the problem.

diaphragm is split in lift pump if it leaks from tell tale hole