RAW image and preview dramatically different

Question

I'm using Aperture 2.1 and am recently finding myself extremely unsatisfied with the rendering of my RAW images created with my Nikon D50.
I am shooting in just RAW (NEF) and sRGB, and am exposing for what I am seeing on the back of my camera. At times I use manual, and others I use the Program setting with a -1.3 exposure compensation since the camera leans too bright; I seem to recall everything exposing correctly until recently. But my memory is shoddy when it comes to how things previously appeared.
The preview before importing shows much the same image that I intended (exposure-wise), as does the quick preview for a second or so during the import process. Shortly after, my pictures all become dull and dark, lacking color and appearing about 2 stops underexposed.
I love photography, but am personally baffled by the issue of color profiles, RAW images JPEG previews, and all that jazz. Nothing seems to explain these adequately, which is why I'm reaching out to those more knowledgeable than me. I am unsure if it's my camera, Aperture, color spaces, or just settings within Aperture or my camera.
Once again, I want to say this is a recent problem, but it may just be that I only noticed recently.

Answer 1

I have exposed to the image on the LCD before and produced wonderful results. Or, at the very least, results that didn't drive me to post on a discussion board.
As per your questions:
--I use 4 different methods to determine how I am exposing my image. In Manual, I use the TTL metering chart in the viewfinder (usually exposing -0.7 to -1.3 so as to compensate for the tendency for the camera's meter to overexpose), the resulting image on the LCD, the histogram, and the in-camera highlights indicator (since, once again, it has a tendency to try and overexpose an image). In Priority modes, I use the camera's exposure compensation setting to set the compensation to -0.7 or -0.1. The resulting JPEG that I see in the pre-import thumbnails and the brief seconds during import are exactly the exposure I want, but when Aperture finishes importing, it's entirely too underexposed. And it's not just that-- the color seems to be worse than it would if it were just simple underexposure... it all feels grey.
-- I use matrix metering most of the time, with spot metering in the rare instance that requires more precise measurement than just tweaking the aperture or shutter up or down a stop or so. Snow or water pictures are a prime example.
Just so we're clear, I know how to use a camera. I know how to expose properly, and in the past year with my camera, I feel I've figured out its quirks. It takes a lot to drive me to discussion boards, as I usually figure out problems myself. But, since the lingo and strange things of digital photography (Color Space, EXIF data, RAW conversion engines, etc.) confuses me sometimes, I am looking for more human help. I haven't had this problem (or this dramatic of a problem) in the past.
The RAW images outside of the last couple of months seem fine (maybe having something to do with the fact that I upgraded to 2.1 from the 1.0 software about 6 months ago and am finally getting back to photography during my summer break). And the JPEGs that I am apparently exposing for are fine as well. It's just after Aperture imports the RAW file that I am having these problems.
Like I said, I want to continue to use RAW. I want it to work like I thought it was working in the past (that is, without major exposure headaches). But I want the image that I thought I was getting. So how do I make Aperture see the same (or nearly the same) image that I am seeing in the JPEG thumbnail? The difference is just too dramatic to pass it off as quirks and be okay with that. And no, at least as the photography part goes, I do not think it's a short between the floor and the camera.