Turn on in your GI reflective caustics often adds only problems. But there is a bypass maneuver, which offered a caustic little bit with a small (about 30%) increase render time but without any stains! To do this, check DE (Detail Enhancement), and to calculate the LC and Ir map in to file with switched off reflective caustics, and before render turn on casuistic and use from file LC and Ir map. Settings of DE will be responsible for the quality of caustics. For a large resolution could be left default value. For the smaller resolution it`s possible to use world and set the desired radius mm. 30-40 mm. quite enough, and fairly quickly.