PowerPC :: Power Adapter Keeps Switching On And Off?

I have a 12" Powerbook 1 ghz (yes, it's getting old) running Tiger. When it's turned off, it charges just fine. However, when the computer is on, the power from the adapter keeps cycling on and off. For example, the lighted band around the power adapter tip switches from orange to off (NOT to green) repeatedly: about 3-4 seconds with power (lighted band is orange, screen backlight is bright, and if I'm in the Energy Saver section of the System Preferences, it thinks it's plugged in) and about 1 second with no power (lighted band is off, screen backlight dims, and if I'm in Energy Saver area, the computer thinks it's on battery power.

This is irritating because 1) the computer won't charge when on, and 2) as I type this, the screen goes from bright to dim to bright to dim etc.

Obvious answer is the power adapter, but I don't have access to another. What does everyone think? is it the adapter or some other problem?

FWIW, the internal hard drive just died and I'm booting from an external FireWire drive. It started doing this power thing a few days before the internal hard drive died.