Insulation, Radiant and Vapor Barriers - Need ideas to fix moisture problem

The house I bought 3 years ago and is now roughly 5 years old has a rapidly increasing bad moisture problem. I have lived here through 3 winters and just this winter all of a sudden there is quite a lot of condensation where the walls meet the ceiling (you can see the wall studs) on nearly all outside walls to some degree. The problem is that there is really no way to see if the blown in insulation is now covering the soffits. The real kicker to me though is why NOW is it a problem especially considering just last winter it became so dry in the house we thought to install some type of central humidifing system. For right now I have been decreasing the humidity of the house with a dehumidifier but I would really appreciate ideas on what to look for to fix whats causing this problem. Thanks.

Your description suggests "Ice Dams". First check you gutters to see if there are icicles hanging from them. Then get into your attic and look towards the outer edges of the roof. Take note of the coloration of the roof sheathing above you compared to these edges. If the sheathing is discolored and/or frost noted at the edges it is an indication of Ice Dams. Let us know what you find.

This generally indicates a lack of insulation behind the moist area. This can occur when loose-fill insulation has settled, leaving gaps. Open the wall and inspect the insulation.
Corners are usually poorly insulated, and they lose more heat than flat walls anyway.

Dust marks on walls that shadow the studs and nails indicate thermal bridging, the cold following the studs right past the insulation. This chills the surface and collects moisture, which catches dust. Put one inch of Styrofoam over the wall, covering the studs, and then finish with more drywall, or add insulation over the outside while re-siding. This will stop the ghosting.