I have a bit of a problem from time to time with Merlin, my latest rescue, especially with my older cat Emo, She is 6, he is 4, and then the others are 14 - 20 months. When he gets bad I put something called Zylkene in his food for a few days and it calms him right down. I buy it from Amazon, it is a milk protein derivative. the 75mg one, it isn't cheap ( about 50p per day) but the return to harmony is worth it. Sue P recommended it to me and I heartily endorse it.
Hope that helps. xxx