Since the N900 is in back-order many places, it may make sense (if you need to go to service) to ask them to try swapping the battery with one that's in stock. The same battery is used by several devices (5230 5800 5800 5800T 5900 N900 X6), so they may have one of those in stock, where they may not have a replacement N900.

Trickle-charging via PC USB overnight may in fact fix the problem. I've seen a post or two on here that indicated that fixed things.

Good Luck!