I recommend cleaning up any flux you have on your boards, especially around the controller data pins on the wii, then test again. I attached a picture of what looks like a possible bridge on the gc+ itself, and a good area to clean of flux as an example.
Is there a way for you to test the gc+ with a different wii or gamecube? If you have a gamecube controller plug lying around, you can solder 3.3v, ground, and data to it and use it to test the gc+. That will tell you if the problem is on the wii end or the gc+ end.
If that's not an option and cleaning up the flux didn't work, I'd recommend disconnecting all of your buttons from the gc+, cleaning it, and then testing it without anything hooked up. If it works, connect one thing at a time, test, one more thing, test, until you figure out what is going wrong. There might be a solder blob on one of your buttons or joysticks.
If portablizemii still crashes with only the gc+ hooked up, you know it's likely a short either near the wii's controller data lines or on the gc+ itself.
