Arduino Pro 5v NewSoftSerial ID12 RFID bug

Hey guys. I have a bit of a strange problem. I've searched this and other forums high and low but can't seem to find a good place to start. I've just finished putting together an Arduino Pro 5v with a SerLCD screen, controlled via SoftwareSerial, and an ID12 RFID reader controlled via NewSoftSerial. There's also a bipolar stepper being controlled with an EasyDriver 4.2. I initially had an issue when plugging the Arduino Pro into 12v power, it would reset continually ( about once a second but it would speed up if left plugged in) unless I also plugged the Arduino Pro into my computer through a dummy Duemilanove (ATMega removed and used as a serial adapter) but I was also feeding 5v to the Pro. If it got both 12v as ext power and 5v for logic, it stayed running fine. Now I'm having an issue whereby the RFID reader works only when it's plugged into both external power and USB. I did make a minor change and moved the 12 adapter from going straight to the Arduino Pro and EasyDriver in parallel to powering the Pro through the EasyDriver because it has a beefy 5v regulator on it. The arduino pro no longer resets itself on external power only, but serial communication is still hampered unless plugged into USB. It is sending something because I had it relay bytes from the NewSoftSerial port to the LCD and it was getting bytes, but ending on "-1" when no card was being read. At this point I'm not sure if that's correct but assume that's not an issue. If you guys want code/schematics I can do that but I figured I'd pick some brains first to see where to start looking. Also, I'm in a dorm and don't have a voltmeter (but will soon :P ) so I wish I had more testing equipment.

Thanks for your insight!