Citroen Xsara Picasso engine fault?

My citroen has just started stalling intermittently, it stalls and it says engine fault on the dash and the yellow engine sign shows up. Yet when it was pugged into a fault code machine, it just said undiagnosed fault so they cleared the fault and the light went away. Just two days later, I stalled again, this time doing 70mph, same again, engine fault but this time it stalled a further 4 times in the next 10 minutes. Then it was ok for two days and the yellow light went off on its own. Then the next day, the same again, its now doing my nut right in, any ideas!!!!