My 3 1/2 year old Chihuahua is marking my whole house. What can I do?

Dear Ed:

I have a huge problem (or it is to me anyway). I have a 3 1/2 year old neutered Chihuahua who is marking my whole house. I assume that marking is different then just peeing in the house. It's not like a puddle in the middle of the room but rather on the corners of everything. I searched your site for an answer and could not find anything on the subject. 3 family members have acquired puppies (and live separate and apart from us) but frequently visit and bring their puppies, or I have to watch them from time to time and vice versa. They are all between the ages of 8-10 months. 2 females, a lab and a boxer, and 1 male shepherd. I caught my dog raising his leg on the 10 month old lab and grabbed him by the scruff of the neck and shook him till he yelped, only because I caught him in the act. So I went out and purchased a black light and lo and behold my whole house has his markings on the corner of everything. I still cannot believe it! I feel I watch him pretty closely. I try to keep him in the room with me (unless he sneaks off and then I'll call him back to me) and I crate him when I leave the house, or when I can't watch him. Please tell me what I'm missing here. Something psychological I'm assuming. I'm bringing a male German Shepherd home in April (at the age of 8 weeks old) and I'd really like to get a handle on this before I do. Please help!! I apologize if this question seems stupid. I pray you can steer me in the right direction by telling me what is going on, and how I can stop this. Thanks so much! Bonnie

PS: By the way I enjoyed your DVD on puppies 8 weeks to 10 months. I plan to use your advice with the new shepherd. And I plan to purchase supplies and other DVDs as well. Thanks so much for your site!!