OK, off the top of my head with all the information presented I'll diagnose:
1 - Dodgy wiring/sensors not hooked up properly/rooted sensor (knock sensors dying seems to be flabour of the month)
2 - Vacuum/boost leak or vac lines not hooked up properly
3 - Fuel pump dying/bad earth
4 - Something else
Hen
PS, this post is only partly taking the piss, the above suggestions are serious, but some more info would be very helpful if you'd like people here to help.