Hmm, to be honest I'm not expert with the petrol Mk2. They do have an ECU so highly likely it's gone into a 'help me help me!!' mode.
Is it cock on 3k rpm that it stops?
I suppose it's possible the rattling you're hearing could be the O2 sensor, but that unlikely to have fallen out or anything, but that might be why it's throwing up a
Have you actually got through flow from the exhaust, and at a decent amount?
Pulled from another forum, does it have TCS fitted? (Long shot)