Always start with your plugs and leads. Give them a change, and inspect the spark plugs that you take out (the back of the Haynes shows you what different effects can be caused by) but look for oil and sh!t down there. If there is, then it could be a couple of things, but let us know and we can get a better idea of what it could be.
I guess the obvious answer would be sticky valves, but this is used too much without a solution being given. More info on the problem could help. Does it suffer a power loss during these revs? Does it happen whilst moving, or at a standstill, or both? Have you changed anything recently? Smoke? When was it last serviced? etc
