Sounds like the battery needs to be replaced to me. They can, when dying out, still show a full charge. As a battery goes, it will take less and less time to charge, and last less and less time. This is due to less and less of the battery being able to hold a charge. Due to this it will tell you it is all charged up when there is way less.
If the tablet is still under warranty, contact the manufacturer about it. If it isn't under warranty, then you can replace the battery yourself, or take it to a tech who can do it for you.