Tombo reeds are notorious for doing this and are therefore not preferred by most overblow players.
Since all reeds have a tendency for torsional vibrations depending on how they are gapped and how straight they are, a fix has been found using beeswax or nail polish.
I prefer the wax because it lasts longer and is easier to rework if something goes wrong.
A small amount of wax is placed around the rivet end so that it is just on the reed. This stabilizes the reed and helps prevent the torsional vibrations.
The wax deadens the reed slightly and makes it respond more directly to the player's breath. An untreated reed will vibrate for a short time after the player has stopped the airflow to the reed. The dampening effect of the wax causes the reed to stop sooner after the airflow has stopped. Though this might be seen as a negative side effect, it is actually quite useful in overblow playing because it shortens the time needed to choke a reed.
An additional bonus to the overblow player is that the wax closes any gap between the reed and the plate around the rivet.
Even when the reed is arced in such a way that it bends down into the plate, it will never fully close off the slot because the reed necessarily curves up to above the plate where it is riveted. The wax seals this opening thus taking away another bit of leakage.
People have asked me what reeds to put the wax on, personally I put the wax on all reeds. The wax slightly alters the tone of the reed and it shortens the decay time of the note. Because of this I put wax on all the reeds to get a sound that is as even as possible on all the notes. However if you only want to use the wax to play better overblows and overdraws then putting wax on the blowreeds that you overblow and the drawreeds you overdraw would suffice.
I have now started to use orthodontic wax, it works slightly better than the normal beeswax and is available online and in drugstores. It is also more convenient to carry around since it comes in nice little portable cases.