1990 cierra 3.3L acceleration hesitates

My 90 cierra 3.3L hesitates when accellerating. It is most noticable when acceleration from a stop when it's cold. I replaced the fuel filter, MAS, oxygen sensor and fuel pressure regulator on the front injector bank. The idle air soleniod and intake air throat chamber area was cleaned. I noticed it would run a little better with the vacuum line to the fuel regulator disconnected so the regulator would increase the fuel pressure. The fuel pressure seams strong, but I don't have a gauge to test it. Can this be a weak fuel pump, even though it runs fine once the engine RPM's are high? There is some device attached to rear injector bank on the incoming fuel line. What is this and could it be restricting the fuel flow?

This sounds like a weak fuel pump to me since the fuel pressure is increased when you pull the vacuum hose. You can rent a fuel pressure guage from most auto parts stores or a tool rental place. This would be quite helpful since it will tell you what your actual pressure is.