1.9dci Scenic Privilege Over Heating

After having our cambelt replaced our car now overheats on long climbs to the point where the STOP and engine stop warning lights come on. Car is fine when travelling on the level. There is also a definitive loss in power as we engage the uphills where the rev counter just drops in 4th gear where before it would accelerate with no problem. Our car was fine before having the cambelt replaced and cam belt was only replaced because we thought it was time to have it checked before it broke. Can anyone send us some pointers on what the cause to this problem may be?..........many thanks