I apologize if this has been mentioned before, but I have had no hope with google, thus far..

My blackberry is only a year or so old (no warranty or replacement options) and has the following symptoms.

While plugged, the phone has no issues. After being unplugged, it will only drain to about 80%-90% until it shuts off without warning. The only way to have it turned back on is a battery pull, only to have it shut off again right after the services have been loaded (startup)

However, after being plugged in for only a moment, and unplugged, it will start up, resume the battery % it had before, and remain on for yet another 10-20%, until shutting off once again.

Sounds like a dead or dying battery to me. Can you try another battery?