vauxhall vectra sri...engine cuts out?

i have a vauxhall vectra sri the engine cuts out when i change down into secound and first sometimes i can catch it before it cuts out when i'm in secound gear or i could be sitting in traffic in neutral and it will just cut out then i have to start the car again...the abs and the tc lights are on all the time and the speedo doesn't work....it's been to 3 different garages and gone on a diagnotics machines but the car can not be read by the diagnostic machine because the speedo isn't working.....please can anyone help....thanks