Citroen Xsara Picasso engine cuts out when braking

I have an intermittent fault, My engine cuts out when braking,all the dash lights come on and i lose the brakes and steering, it usually starts back up first time, this problem has been happening for about 18 months to 2 years now, it happens randomly about every 4-6 weeks, i have had a laptop diagnosis and there was no fault codes coming up, just a lambda sensor running below standard, i replaced this part(on top of manifold) but the fault occurred again, can anyone diagnose this problem for me as i am getting very frustrated by it, it seems to happen when i am braking and always seems to be when i am on a downward slope,only once happening whilst on a flat road.