Check to see if it hold pressure when you pinch off the fuel return line. If pressure still falls, replace the spider injector assembly. If pressure does hold, then it is a faulty fuel pump.
99% of the time it is a faulty spider injector assembly. In order to replace it, you will need to remove the upper intake manifold.
The directions I gave you should get you thru replacing it without any issues. If you run into a problem, re-open this questions and I will continue to help at no additional charge