Time to replace my battery?

I have a Lenovo ThinkPad X61 that I bought a little over a year ago with the 8-cell (extended life) battery. The battery had worked quite well up until recently; I would get up to 6 hours of use from a full charge.

The battery now seems to be having problems. It will run down normally until it hits the 70% mark. All of a sudden, however, the power manager will start showing the battery at 4% or 5% charge. And then the laptop will enter hibernation mode, of course.

Is the battery dead? Time to replace it?

It's Lithium-Ion, so I'm guessing this is not a "battery memory" issue?

Also, an official Lenovo 8-cell battery will cost me $180 from Lenovo. Does anyone have suggestions on how to tell if a online retailer (1) is selling an official Lenovo battery, versus a knock-off? and (2) is selling a refurbished battery, as opposed to a new one?

Any online retailers you would suggest?

Brian