t5 shuttle

[linkscabs]

I had my t5 transporter 55 plate into vw last week it kept jumping out of reverse gear,they ordered a new gear box,then they decedied it needed adjusting with a tool that they didn`t have ,they got the tool adjusted the gears and it was alright till yesterday,jumping out of gear again.Do I now demand a new gear box ?.As this is a working car and I can`t keep handing it in every few days.Or is there something else that an be done to solve this problem?.
Regards
Mike