Do 532nm crystals go bad over time?

You might want to try other batteries. The 260 I have will peak over 300mW on some batteries. Yet it won't reach 260 at with others.

Yeah I mentioned power. Your battery might be getting old. What I'd do is mount it in a rigid something like a rail, turn the current to minimum, then give it a fresh battery and turn it on. Then slowly measure power vs current. Then put it on the most efficient setting.