bmw 320d problem when i floor it?

i have a year 2000 bmw 320 diesel, when i floor it in 3rd, 4th or 5th gears it seems to give me lots of power to start off with then holds back (it feels like ive taken my foot off the accelerator) then when the revs go higher it gives me more power again, if i dont floor it, it seems to accelerate smoothly. can anybody help me with what the problem is please