Peugeot 307 automatic gearbox erratic behaviour

My 2004 1.6 Peugeot 307 will be driving fine one moment and then the next thing the revs shoot up, the engine whines really loudly and the car will not engage the transmission correctly....This occurs more often when the car is put under load when going uphill.....If the car does move then it is sluggish and the engine whines when the transmission changes up or down....Switching off the ignition for approx 10 minutes usually resolves the problem....There are no warning signals on the cars display when the problem occurs & the car was recently in a Peugeot dealership and no error signals were detected....As this problem is intermittent I am beginning to move away from the idea that it is purely a transmission problem, could it be any of the engine management sensors?...Any help would be appreciated.