Citroen Zsara engine management light intermittent?

I have a Xsara 2.0 Diesel 90BHP on a 51 plate and sometimes the management light comes on and I can only get half throtle then if I stop on level ground for a few minutes it clears then all is well untill the next time. It happens more if I park overnight on a hill. I really need to sort this out as I have a small baby and don't want to be stranded if possible. Thanks.