UK Car Repair: car keeps cutting out, vauxhall vectra, poice

Question
my car keeps cutting out. had it's mot last month, and it wasn't picked up. they passed it. it cut out before the mot.
it's everytime i get to a junction or roundabout.
been stopped by the poice because they saw me at a junction.
it's a w reg vauxhall vectra.

why would they give it an mot pass if the coppers are obviously not happy with it?

many thanks

Answer
Reliability is not an MOT failure, neither is it illegal.

Best have the cars diagnostics run.

Could be any number of faults,

how long does it cut out for?


(how unhappy were these coppers about a breakdown, they should help you ideally not be unhappy)