Another solution is to use the Ctrl-Alt P diagnostic built in all the I-Pac.
Open up the notepad/wordpad and press Ctrl-Alt P.
It will give you some options, press t for "test" and if you have any short in the wiring, it will let you know. Most likely, it is a short of some sort. I had this before and it is also discussed in the Ultimarc Trouble FAQ.
From Ultimarc...
ADVANCED TESTING (optional)
If in DOS, enter COPY CON to suppress the command prompt. On a keyboard plugged into the pass-through, press CTRL-ALT-P. (press and hold each key in this order). Enter "T" in the menu to enter test mode. All pressed buttons and joysticks will be displayed on the screen. The input will be shown, followed by the code which is currently assigned, followed by the shifted code, if one is assigned. Test all buttons and joystick switches. If all OK reboot to exit. Now ready to play!