The reason that I suggested the battery is that is is extremely old (AST went out of business years ago - at least five or more years to my knowledge.) and a likely candidate for the problem. It could be so bad that it is dragging down the power from the mains PSU. You say that they battery is fully charged but how do you know? Typically a bad battery can look as though charged but the charge only lasts for a short while. (memory syndrome)
Even though AST are out of business you may find some useful stuff on www.ast.com as one of their associates has that site now.
Terry
September 2006