flat spots and black smoke

Hello,

I've got a problem with a Skoda Octavia 1.9 tdi, 110 bhp, AHF engine code, 99 reg. I bought it in april 06 knowing not anything about turbos and after driving it a few months I realized that I have a very sluggish car. After searching over the net about possible causes I took the car to a chap and had it VAG COM diagnosed but no fault popped up. But when the chap took the car for a short spin around the block he said that it is the MAF meter which needs to be replaced. After cleaning the old one without any result I bought a new one and I fitted it. When starting the engine I noticed that a lot of smoke coming out of the exhaust for a few secs but I drove away for a spin on the motorway. It was feeling very briskly in 1st and 2nd gear but the engine pulled in an intermitent way which is still happening. When in 3rd, 4th and 5th it is flat around 2000-2300 revs and I need to put the foot down on the pedal in order to have a brisk release of power. When this is happening black smoke comes off the exhaust which disappear after one sec or two If I am accelerating further. It is ok after 75 - 80 mph, no smoke, decent power. If going uphill it is worse. When caught around 2000-2300 revs uphill the car smokes even more which is annoying when you can't go faster and I feel a bit guilty for motorists behind. I think that the smoke occurs when the turbocharger tries to kicks in but it is very frustrating because I need to accelerate hard in order to go over the flat spot but it seems it is losing power having the pedal pressed in the same position and I need to accelerate more in order to keep a steady speed. Uphill is the worst I have. If going fast it is ok but at lower revs the loss of power occurs and the smoke appears when trying to regain the power.