Sounds like it needs a new board.
Looking at the Ion schematic,
, there isn't much that can be making this happen.
From the schematic, the signal wire to the main board is high (~5v) only when there is something blocking the eyes. But, that 5v is through a 24.5k ohm resistor, so max current if that signal wire was shorted to ground is 0.2mA (0.0002 amps), not near enough to cause this problem.
So you're down to either the chip thinks the power button is being pressed (possibly when something jostles the gun), or something is shorting out the 5v supply and sending the voltage regulator into thermal shudown (which looks like battery removed and re-installed to the chip).