I do not know about this PARTICULAR lens but wobble on the barrel usually indicates that the robust side screws (usually under the aperture ring and/or focus scale ring?) are coming loose. It you can get to them (again, this particular lens I am not familiar with) all you do is tighten them. They are usually underneath all rings, including that focus scale ring.

Usually, this is accomplished by loosening the tiny set screws holding the focus scale or aperture ring and then slide off all rings until you get to the bottom bare metal where the (usually three) robust screws are that hold the barrel in place. This might necessitate removing the mount (which is not difficult to do but work with a white cloth underneath so that you lose NOTHING!) - David Lyga