Wanna,
Could be bad paint? Or, could it be what you're using to wash the car? I mean don't get me wrong, I know you're a pro, but I'd say having to wax every month is a bit much. maybe your soap is stripping it? It's a guess?
I have a buddy with a black 99 X Limited that he puchased it late 1998. The finish still looks almost showroom new! As far as I know, he waxes twice a year, the end of fall and the beginnig of spring. I have seen him do a light detail spray wax, if he washed it and didn't use his choice soap, or if maybe it streaked when he dried it.
Also, my co-worker's husband has a black 97 F150, he has it prof waxed and buffed once a year. Other than that, he says he rarely hand washes it, but when he does he only uses one kind of soap. For the most part, he does the touchless car wash thing. The finish still looks great on it too.
Both of the guys above, and I (even though my X is white this is all I ever use even on my black Honda Accord) swear by "Mother's California Gold". We use the clear coat wax, detail polish spray, and soap.