By pass valve

Hi guys I have a 2005 jcw and I was wondering how can I check if my by pass valve is working properly?! As I have this kangarooing or yo yo effect at high revs under hard acceleration and I think it's the valve but no one at a garage seems to think so! My CAT is noisy and I'm told to get a new one and that this could be my issue with the weak start up and hesitation but I think there is more to it.
Please advise many thanks