Get the car jacked up, get under it and have a look just behind the bumper on the passenger side. There'll be a thick hose (as thick as a clenched fist) running to the airbox and then into the turbo (i think) - this pipe commonly rubs on a bolt on the gearbox housing and puts a hole in it.
You will probably have to take the hose off to check properly as you'll be looking up from beneath and the hole will be near the top. Silicone replacements are best, you can get them from autolusso for about £70 i think. Rubber is cheaper of course, but won't last as long. Depends how long you wanna keep the car i suppose. Some have bodged by replacing the holey section with a run of alloy pipe and some jubilee clips if you're feeling the pinch a little at the moment - shouldn't cost more than a tenner that.
The hole can cause the black smoke, due to the excess fuel burning off because the engines not getting as much air as it thinks, and can cause that odd hesitation-esque feeling. Normally this is accompanied by a whooshing noise of air during boost mind, but if it's only small it may not be audible over the engine noise yet. You could always get someone in the car to rev it to about 2k+ while you have your head on the ground to see if you can hear/feel the air whooshing, it's quite distinct.
Last edited by diddums100; 04-07-14 at 13:28.