This issue has to do with the capacitive keyboard ground scheme and can be triggered by several things including the humidity of your environment. Often placing a finger on the edge of the circuit board can help stabilize this effect.
Make sure all the keyboard resistors are the correct values and soldered properly. Too much or too little solder can actually cause these types of issues. Proper soldering is important for stability.
This type of issue is commonly related to the type of power supply you are using. You can experiment with a different power supply. …Or try a (brand new) battery if you are using a plug-in wall wart or try a wall wart if you are using a battery.
-Brach