2006 Renault Clio auto gearbox problem

Driving normally, 40 - 50 mph, sudden "crunch", read-out on dashboard display reads "check gearbox" - drive slowly to local mechanic ( no other noises ), garage doesn't find anything wrong ( on diagnostics or road test ) - pick-up car following day, two minutes down the road ( same road same speed ) "crunch" - feels a bit like a manual that has been put into reverse for a split second - wassagoingon-cosiain'tgoddaclue. Thanks