Is this the original OEM shifter? It should be snug tight without excess movement, make sure the metal ring is clamped tight around the plastic fixings of the knob, you can do this by gripping on the gearbox base with your hands and pull it up to expose the lever and then use pliers to tighten the metal ring.
If it's an aftermarket knob then there could be the possibility the plastics is not being thick enough or the metal clamping ring is too big causing it to move around the lever. I had a €20 knob which looked nice but had poor fitment - the plastic tabs was very thin and flexed about which consequently made gearchanges very slack. I decided to get a genuine Edition 35 gearknob in the end and it fitted tight as well as solving my slack gear changes.
