vectra 2.2 sri keeps loosing power?

Have jsut bought a 04 vectra and once you get to 4thousands revs or press the excelerator quickly to over take the orange light with the car and spanner comes up the car looses power and goes into limp mode car is also miss firing a bit to. the garage don't know whats worng with it the computer isn't picking up any faults either,they chanhged the fuel pump but it wasn't that and the fuel filter is new. any ideas of what could be wrong.
many thanks