We have two 5-year-old male indoor cats, brothers that were both fixed
at 6 months of age. Over the past 6 months on 3 occasions one of the
cats has become very dominant and has aggressively attacked the other.
So far these episodes have lasted for about 24 hours and then
everything goes back to normal (very friendly). Is it possible that
one of the cats has become un fixed, or maybe that he never was fixed
properly?

Any help or ideas would be greatly appreciated!!