Have you tried a different power adaptor? Any Duo (will run the machine, though will NOT charge the battery), 1400, 2400, 3400, G3 or clamshell iBook adaptor will work.

I've had my PowerBook 1400 since new, and I remember in its first year, having this very problem, and I remember that it definitely turned out that the power adaptor was the problem, which was replaced under AppleCare.