http://cyclepathnw.com/
They specialize in custom builds. While not DH in particular, I can't say I've heard anything but good things about their service. Just up MLK a ways N of Burnside. A buddy bought a Titus El Guapo there (custom build) and I've had a wheelset built there.
http://www.ride-this.com/index.php/company-info
These guys I've never used for service, but they are a good local shop, too. I'd give them a shout and see what they can do... I've used them just once for purchase, but had a good experience, and they like big bikes. On the other side of town (Milwaukie), though.
Just a couple that
I'd start with, but there are plenty of compentent wrenches in this town.
YMMV.
PS- What's the frame/build?