2011 Vauxhall Insignia car keeps cutting out when driving..!?!

I have recently purchased a Vauxhall insignia 2.0 CDTI. At first all seemed well when I got the car home on day number 1. after around 3 hours the car just cut out. At first I thought I've just stalled the car (with my foot on the clutch?) So anyway I tried to start the car back up and it turned over but it wouldn't start.. I left it for around 30 minutes? and then it finally managed to start. Around a few hours later it happened again so i repeated the same process. I sent the car to the seller and he assured me it was fixed.

I've got the car back now and it seems to have gotten worse and it is happening every 30 minutes the car doesn't seem safe to drive as because when it does happen the power steering deactivates. :(

I bought the car with 1 months warranty the seller got his mechanic again and had it checked. This time they said "it maybe the oil level is too high which will cut the car out," to which he then drained the excess oil.

I waited for an hour or so to let the oil set and then tried taking it for a good run.. around 2 mile into the run it happened again. i just managed to get home after waiting for an hour and don't really trust the seller or his machanic's.. I need to either solve the problem and address the seller? or send it back :( but really i wanted this option to be the last resort?

any idea's please?