The most important question is how long you have had the battery. Ego twist are notorious for being underspec depending on how credible the company you brought it from is. A lot of the clones have very....frugal....batteries which mean they dont posses the typical 300 recharge cycles. Your batteries characteristics sounds to me like it is at the end of its life. This is very common.
I used to work for a very big company that sells ego batteries, and one of the lies we have to tell the customers is that it is their fault thier battery isnt charging as much as there are "too many factors" that could be blaimed for why their battery is dying quicker and quicker. The reality is, its just a crappy battery. The Quality Control in china is shameful.
On a good note, it's just an ego twist. 20 bucks will get you a new one, or if you
fasttech it, you might as well buy a few. Personally, I think its time you upgraded to something better. Ego batteries are useless money pits.
Hope that was helpful.