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[Jet Williams]

I've recently had a new downpipe fitted for the Mini to pass it's MOT, and ever since, it's had a very noticable flat spot between about 3800 and 4000 rpm. When it hits 3.8k, there is a small but obvious decrease in power, and the power then comes back again at 4k.
To fit the downpipe, it was necessary for the manifold and air intake/throttle body to be removed in order to get a broken stud out of the manifold.
Also, the car has been idling rather high (about 1500rpm) on start-up for about 5 minutes, at which point it settles down a bit. Again, this has only happened since the downpipe has been replaced.

Any ideas on why this might be?