Had too many objects asking other objects what they were doing, so I moved timers out of towers and into their effects. While doing so, I found an error where there was just one master spray effect in the entire game, and all of the slaves were looking to it. This wasn't a big deal as long as the slaves all pointed at different towers, they operated reasonably independently (but there were minor issues here and there, which is the reason for the refactor).
The minor issues were probably only graphical: the splash effect was only displaying in a minority of affected squares (although it looked like it was killing correctly). All that's fixed.
Although it looks like the game no longer kills the original source. Need to fix that. Maybe I can fix the dropped sources problem as well.