I need some help badly. I've got an 01 s3 which is ment to have been remapped. It's had a recent service, new stat and cts, green Cotten panel filter. The spark plug gap I think is at 22 thou. I bought a milltek cat back 2 weeks ago. I knackered the lambda while taking my old exhaust off. When driving after about 4000 rpm it looses power badly when I am heavy on the accelerator. At first I thought it was because of the lambda it was running rich and over fuelling. I replaced the lambda last night and it's still no better. It's like it starts to boost nicely, responsive throttle, then gets to around 4000rpm and doesn't have comparable power to the first 3500 to 4000 rpm. No faults are found so I don't know what to do. Any help will be much appreciated.