Hi Sarah and welcome, my auto has done 88k now and just drove up Sutton bank in yorkshire 1 in 4 no probs with mine. Have you had checked the fluid level. These gearboxes are fairly intolerant of low fluid level. As you just had a new one fitted maybe they couldnt quite fill it properly or it has a minor leak ,I would think the level is the easiest place to start. Hope this helps