Aha! So that's the secret! 
Thanks a lot for the information -- I haven't looked into the USB code yet, but this suggests to me that the Arduino must apparently wait for a "successful transmission" confirmation from the PC that the keystroke was sent before it exits out of the UsbKeyboard.sendKeyStroke() function and continues executing the code. So, my arduino actually wasn't freezing, it was just never getting the response that it was looking for to continue.
This may also explain why committing to solder made things work out better for you. You probably had a loose wire in there somewhere.
Thanks for your help, huntd69 and Nick! Looks like I'll be placing a component order tonight XD.
arduinoRobo