Anyone experience this with the Mac?

I have an aftermarket power supply for my iBook (a Macally). Recently, when I have it plugged in, it will often cause the battery indicator to flick between "Calculating" and the amount of power my battery has left, causing the whole screen to flick between bright and not-as-bright. Presumably it also makes charging a lot slower - essentially, the computer thinks that it's continually switching between being plugged in and being on battery power.

Thoughts? Whaaa?