I'm running 19 miners currently on the Pi and it has not had any stability issues. As you can see in the screenshot below, system load is under 0.1, cgminer processes are mostly sleeping. Some differences from your setup: 1) I'm using scripta and a precompiled cgminer posted by someone initially on litecointalk; 2) I'm running two separate cgminer instances (10 and 9 miners). I would suggest for you to try the precompiled cgminer and see if that performs better. If that doesn't help I would start narrowing the problem by disconnecting half of your miners to see if there is one malfunctioning device that might be causing cgminer to misbehave.