Toddles,
I don't ride a custom frame but I find that my proportions are fine for a couple of off the shelf frames, but not all. I ride Rivendells now, because they are the best bikes for me and I find them to be comfortable. I can ride my riv all day (and have) and I don't get any discomfort from body positioning at all. The bikes and the way I have them set up are just like a custom made frame for me. I could never get comfy on the Trek Madone I had, nor could I get comfortable on the Jamis Aurora touring bike or the Lemond Zurich I really wanted to fit me. I tried everything but just can't get it. If I didn't find my Rivendell and it didn't fit me the way it does and give me the ride it does, I would have been searching for a custom frame.
If you have weird proportions or just cannot find the right bike, a custom can probably help with that. With you living in Washington and near Oregon you are surrounded by a pretty good number of custom builders and you can actually get a proper fitting done by them in person if you decide to buy. You could try Mitch Pryor
http://www.mapbicycles.com/, Co-Motion Cylces
http://www.co-motion.com/index.php/s...general/custom, Boxer Bicycles
http://boxerbicycles.com/ and I am sure there are a ton more. If you have tried to get comfy on your bike but just can't, a change to some other bike may allow you to ride farther and faster. It did for me.