Hello david. It is a somewhat delicate box and more considering that it surely has a good kilometers traveled. The problems you are having are typical of internal wear, fatigue and breakage of internal parts. To solve the only option is to disassemble the box, disassemble completely and check piece by piece. You can find gears with damage to the entrance of the teeth, brakes and synchronizers worn and / or broken, play on the forks and other parts such as bearings and springs with damage. They are somewhat complex mechanical parts and if you are going to do it, it is advisable to have on hand the phone of someone who knows enough about the subject since a small error like a badly placed spring can lead you to assemble and assemble to discover that it does not work. A hug.