Loss of power?

I have a 2.0 dti Zafira which on occassions whilst driving just looses power and the light with the car with spanner going through it comes on. Whilst on the last occassion this happened I was struggling to go up a hill and thick black smoke was coming out. This problem then seems to go once the car is left off for a while, this time varies. I have had it on the machines but it shows no faults read or stored but this has always been when problems gone and no lights are showing.
Any Ideas!!!!