my battery on my ipod is fully charged but after 10-15 min it will say battery needs charging then it will have a essage to restore ipod

My ipod classic is having problems even when the battery is fully charged it will suddenly say my battery is drained then when I connect to a power source it will brng up the message "connect to itunes to restore ipod" if I immediately unplug from power source and let it restart it will show a full battery again. BTW when it first brings up the battery message the ipod will constantly restart until I plug it in to a power source. It has been doing this of and on for the last 2 years the ipod is only 4 years old.