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!!!!

Mods
March 2009