Maybe the electrical connectors are somehow corroded after several years, specially if the plane was stored in humid conditions.
I would check all the connections first.
Also, the battery could show good voltage and be weak to some amps.
Check for black wire disease.
Check these links:
http://www.hangtimes.com/black_wire_disease.html
http://www.hangtimes.com/rcbattery_faq.html
http://www.hangtimes.com/redsbatteryclinic.html