316i engine problem

[Dave]

Hi, Have a realy anoying problem with my K reg 316i. Each time I go up the gears I loose power at about 3-3500 revs, its like when you hit the 6000 rev limit. The engine rattles and holds back until I change up a gear and the revs drop, it is most noticible when going up hill or pulling away at a junction. Any ideas would be of great help. Thanks in advance.