steptoe, if you didn't touch the clutch it probably isn't the problem. Did you by any chance, replace an aluminium key with a steel one? And is this key substantially bigger than the original one? Depending on the diameter of the shaft the key is in, you may have created a big enough unbalance for it to be a problem. Remember, steel weighs three times as much as the same volume of aluminium.
It is normal with forced vibrations that the amplitude varies as you increase the speed, due to various parts of the system resonating. Having it feel smooth at high speed does not mean it isn't a problem.
Most likely the problem is in some part of the system that you have altered. It may not be the key - but it isn't likely to be a part you haven't touched.
