Cats fighting

[HOGEPH]

Our collection of 6 cats have generally got on fine, but in the last six months, the 3rd oldest cat (4yrs) has been having a go at the oldest one (9yrs), but not all the time, they'll eat together, occasionally share bedspace, and tolerate each other, but then a full on hissing match can start and the younger one will attack the old cat, who runs for his life.

We've tried Feliway, with no joy. Any suggestions? They got along fine for the previous 3 years.