Santa FE SM (2001-06) :: Air Conditioner Intermittent / White Mist Will Come Out Of Vents

I'm having intermittent problems with the air conditioner on my 2004 Hyundai Santa Fe. If it matters, it's a 2wd, 3.5 liter model with about 66,000 miles. Below I will post a description of what's happening, as well as some other things that I've noticed and things that I've tried (these may or may not be relevant to the problem though). OK, here's what's happening:

I first noticed the problem back in late July and since then it has been happening more frequently. So far the problem only occurs after I've been driving on the interstate, although I'm not sure if it's a matter of my speed, or just a matter of how long the a/c has been running. Typically I only drive 15 minutes at a time unless I'm on the interstate where I would typically drive for a few hours or so. What's happening is that after a while (the time varies from 20 minutes to an hour or so) the flow of the air conditioning coming through the vents slows down to just a trickle. The a/c is still really cold, but it just barely comes out of the vents, no matter what speed I set it on. Here are some things that I've noticed (and a few things that I've done to try to repair it):

1.) I read that it might be the cabin air filter, even though normally Santa Fe's aren't equipped with them. I took the glove box out, and found the spot where the air filter normally goes and there was one installed (we bought the car used). I took it out and it was disgustingly dirty - a bunch of dirt, some leaves, sticks, etc... I just took out the filter completely and didn't replace it.
2.) While I was looking at that air filter, I also noticed that the box right to the left of that filter was wet with condensation. This was right after a two hour drive with the A/C on maximum cold all the way home though, so I don't know if that is normal or not. It wasn't dripping on the floor, but there was noticeable condensation on that box (I don't know what that "box" is).
3.) The problem will sometimes go away if I turn the air conditioning completely off for about 5-10 minutes. It seems that the longer I leave the a/c off the better chance I have of it working when I turn it back on.
4.) When I change the fan speed on the air conditioning I can hear the blower motor running at different levels. It almost sounds as if there's something in the dash that is clogging up the pipes. I can hear a whistling sound when the blower is on the lowest speed - when it's up any higher than that I can't hear it, but that's probably just because of the noise coming from the blower itself.
5.) I had the problem today, and I stopped at a gas station to get a drink. When I got back into the car I heard the sound of water trickling from underneath the glove box. It was very quiet, but it was there.
6.) When we have this problem, it happens on all of the vents. It doesn't matter what vent you choose for the air to come out of - the flow is just a trickle.
7.) It doesn't matter if it is heat or cold - the flow is very slow on both of these.
8.) The a/c is still ice cold during all of this. It blows colder than any other a/c that I've ever had.
9.) I noticed that sometimes while driving, a white mist will come out of the vents. Sometimes we have the problem above after we see the mist - sometimes it still works fine though, but I did think it was odd that the "mist" was coming out. I'm not sure if it's water vapor or what, but it does go away if you turn the temperature control a little to the warm side (even just a notch or two away from max cold).

Originally, I had thought that it was the blower motor and I was just going to replace it, but then I started thinking about it and don't think that it is the blower motor because it seems to be working well most of the time, and even when I have the problem I can hear it running - again it sounds like something is clogging up the a/c system somewhere between the blower and the vents. I also read some things about the blend doors, but I'm not sure if that's it or not.