Originally Posted by Breadfan

Hmm. I had similar issues when I was tuning for a different intake. Before I was tuned the car would skew the long term fuel trims to accomodate the different intake. These remembered long term fuel trims would cause the car to stumble on cold start until the active short term trims kicked in to adjust in real time. Perhaps pulling the fuse for the computer to reset it now that you have swapped back to stock would help? The surging at part throttle seems to suggest something else though. Maybe bad purge solenoid? Maybe weak bypass valve? Hopefully, someone more expert than me can jump in here.