The only time Iv'e seen it in TS12 is on large or detailed routes, putting a significant load the gpu and or cpu. In my case I had to delete about a hundred base boards. I model terrain w/TransDem, and have a habbit of generating routes with too many boards; trimming later.
After I trimmed back the extraneous boards, I packed the route in a cdp, deleted the route in Surveror, and then reinstalled it via the CMP. That seemed to solve the problem pretty much. The route will still drop a tile occasionally, but toggling the Grid button on and off seems to solve it.
Another ploblem is distorting tiles when moving objects around. This is an issue with mouse sensitivity, where the mouse may grab a tile and displace it. Setting the mouse sesivity way down should correct this.