[Dancan PMd me with the information that this has been an ongoing problem with their cat for years... I thought I'd share my reply here, in case any of it is useful to others.]
A few ideas to try:
See if you can find a vet who uses both traditional and herbal approaches, and take your kitty in for a thorough going-over. It's possible there might be some kind of treatable medical reason for this behavior, which would be a great relief to you AND your cat! But if there isn't, this kind of vet might be able to suggest some sort of herbal therapy that could soothe and calm your kitty.
Personally, I've had some degree of success with certain essential oils that can be massaged into the tips of the ears, particularly one called Rescue Remedy that's widely used with cats -- you can find it in the herbal section at a health food store.
Also, I'd suggest you go to valleyvet.com and order a Feliway Comfort Zone diffuser and refill (not the spray -- it isn't nearly as effective). Feliway is a synthetic feline pheromone, and it works wonders in keeping our six kitties calm. (We just discovered this excellent source, which is far less expensive than local pet supply stores.)
If all else fails, you might want to see about Soft Claws, those blunt nailcaps that fit over the ends of your kitty's claws to minimize the damage he does -- but I wouldn't put him through that additional stress yet. I'd concentrate for now on trying all the methods above first.
And finally: do you know Hissy? She's the guru of these issues, and if I've missed anything you should try, she'll think of it. You might try PMing her.