it will accelerate gently, but dies when you open up the throttle?
That must be fuel starvation i suppose, maybe a fuel pressure problem, faulty rail sensor, maf etc but the cat wont cause that, its impossible. The lambda could reporting overrich mixture when its not and the ecu has "leaned" off the fuel by adaptive trim.
Try resetting the ecu using the battery off method described in other posts. You must follow the exact sequence for the ecu to relearn the adaptations and return to factory settings. Search the forum for "ecu reset" and try that first.