wild which soft line are you talking about ... back by the fuel filter or up front at the high pressure line right before it goes into the throttle body
I was talking about the one in the front,
but either will do, the pressure drop would
indicate a weak fuel pump or bad/plugged filter,
but since you changed the filter, I don't think that
is it, you did change the filter right

I think you should replace the pump, you can
use a bosch external pump, and it will bolt in
the the banjo fittings at the filter if you do it
right, I added a bosch pump when I needed to
boost fuel pressure, I ran both pumps from the
same power feed, you could do the same, and
a used wrecking yard pump would be much less
than the new unit which I hear are about $100
Wild