i have a very simalar problem to montiforestcat, i have 2 brothers simalar age who have always got on and now suddenly if 'bailey' comes into the room 'dooley' growls at him and then 8 times out of 10 a fight starts, i also have a large enclosed garden with trees so all 9 of my cats have lots of space to be on their own, 'bailey' doesnt want to fight and wouldnt do if 'dooley' wasnt growling at him, i have a suspicion that they were playfighting and as 'bailey' doesnt know his own strength he would have hurt 'dooley' they still eat together though and theres no growling then,
years ago i had 2 cats who were inseparable and suddenly they too 'fell out' i let my sister take one of them to her house for a week and when she came back they were inseparable again and the problem never reoccurred, i would do this too with the brothers but my sister lives in england and i am in spain but i am going to give feliway or zylkene a go and see what happens.