Well, after a while, rechargeable batteries can end up just holding a 'surface charge', meaning a charged area gathers near the anode and cathode but the rest of the cell is pretty much dead. The charger will still see it as charged, but it won't be.
When that happens, the phenomenon is, you get a short burst of nominal current, then it dies off. So it's best to eliminate that possibility, even if the charger is saying everything's A-OK, (i.e., 'light going green') - just to cancel the possibility of that variable.
Mattman, yours i'd guess is the batteries.
If Lasermodder's is dying progressively every time he turns it on, that could be catastrophic failure of the diode and it slowly going LED. A passive component slowly losing conductivity (SMT capacitor?) could also be causing what you're seeing, Lasermodder, or, actually -- a melting/charred/over-currented spring or switch. It's hard to say without some troubleshooting. If it's an "end cap clicky" host, (I believe the Power Rock is) try bypassing that variable completely by taking off the endcap and shorting the battery terminal to the casing. If it's still dim, then the driver would be the place to look next.
The thing to remember is, don't worry too much: most of the time this stuff is replaceable.
