love/hate relationship

Our two cats have a love/hate relationship with one another. Thomas, our 3-year old neutered male, is friendly & loving, but afraid of strangers. He was two when Sam arrived on the scene. Sam is now one, also neutered, very outgoing & loving. They get along well at times, sometimes even grooming one another lovingly. However, at least once a day, one of them (usually Sam) will chase the other aggressively until he's caught & will then try to hurt the other (usually bites the back of the neck). They are both kept indoors. We do allow them to go out into the attached garage for short periods of time each day to give them a new place to investigate. I am wondering if this is normal behavior for two male cats that are kept indoors, or is there something we can do to help them work through their issues? Your help is much appreciated!

I would invest in a Comfort Zone Room diffuser by Farnum Pet. They are very effective and can literally be found in any good pet supply store, or you can order them on the Internet. Once it is place, it takes about 3 days for it to have a calming influence on the two at war, but it does make a major difference.