Xsara Auto drops into second gear!?

My 5 year old Xsara Diesel Auto Estate has a habit of dropping into 2nd gear without warning at speed. Two lights flash on the dash and the only way to get the gearbox working again and to stop the warning lights is to switch off the ignition and restart! The car is under a bought warrantee but the fault did not occur when tested by their engineer. Any thoughts please?