Any ideas on my Zafira?

I have a y reg zafira, until recently it has ran great but it now cuts out when i am going around 60mph and then when i have to break quickly going down to around 45mph it then cuts out. But will start straight up after.
Would be great if anyone has any ideas what it could be.
Had a diagnostic test and it showed that the back of my exhaust was blocked this has now been cleaned but it will still cut out when going high speed.