MakerBot Print is our newest print-prepration software, which supports native CAD files and STL assemblies,
allows you to interact with all your printers via the Cloud, and many other exciting new features.
I am not going down from 12V with 28BYJ-48. When running bipolar you joined two coils together, so it have twice that big resistance. Means it should run at 10V instead of 5V and 12V is not too far from it.
What you describes may be caused by uninterrupted trace at the PCB of 28BYJ-48 when doing the bipolar hack. At 5V the motor is still somehow running, but at higher voltage, this short start to throw over current fuses of the stepper driver.