Lithium Ion batteries have a definite lifespan of only 2-3 years after which they really start to screw up - even if you don't use them. this is due to the chemical breakdown inside the cell.
Most likely time to get a new battery.
I recently picked up a new battery from a vendor that someone else talked about in this forum (MegaCapacity I think) and it's made by Sino. Seems pretty good so far.
Found my post - got this baby:
High Capacity Battery For DELL Axim X50V Extended with back cover Li-ion 2250Mah - Axim X50V
Took 1.5 weeks to get to Toronto from Hong Kong.