Q. My cat has recently become very viscous for no apparent reason no change in routine food people ecany ideas?
A. This sounds like it could be stemmed from a health issue or a behavioral issue. I would always start by ruling out a medical explanation (ex. Pain) by getting a check up. If his health seems normal then I would move to treating a behavioral issue. It could be territorial aggression causing the issue. I would try to get a cat behaviorist called in to evaluate the cat. Aggression can be influenced from MANY things; changes in the household, addition of new people or pets, hormones (but your cat is neutered so can likely rule that one out), stress, fear etc. They will be able to work with you to try and find the route of the problem and a solution. Aggression is very serious and needs professional assessment and treatment. This is not likely something you can do alone.
If you can find it online or on TV "My Cat From Hell" or " Psyhco Kitty" has a few episodes with quite aggressive and aloof cats with strangers. They offer some really great tips to work on these issues and will help give you some background knowledge as to why this happens and how to work with it.