Overboostin Problem

sup guys? my car is overboosting my wastegate (internal) is set for 5.5 psi... ever since i got custom 2.5" piping its been boosting 7..-9psi. funny thing was that it wasnt doing that when i first got the piping done..only like about a week or 2 after. i heard that internal wastegates need backpressure to function properly..is that true? heres what i think may be wrong:
1. wastegate is getting stuck and i have to spray both sides of the actuator rod with lubricant to get it to move freely again
or
2. bigger exhaust diamter causing turbo to overboost/spike..and have to put my stock exhaust back on..

at 5.5psi the stock exhaust should be fine right?
any help/opinion would be appreciated guys..thanks