The pressure drop just means the pump can't keep up. Change to a new pump or a dual setup. Stop chasing your own tail just because you assume this pump from a friend is good. In general, try not to assume anything is working properly until you rule it out conclusively.
It sounds like you need to get your car to a good tuner who can go over the setup, make some necessary changes and spend some time to get it running properly. Your friends might know some things, but properly setting up and tuning a setup like this is not easy especially if you haven't done so before. Hell, just keeping a car idling consistently and not stalling with the ID2000's requires a couple tricks that most amateur tuners will not know about.