Sean, Do you want or need the space up there? While a hip roof design can be easier to ventilate and insulate than other roof designs, it will never be as efficient as leaving the space as a temperature regulator for your house.
You really have to go back to square one and figure out how install a total insulation blanket between living space and the roof/outside. The next part of your figuring has to be how to get hot/cold/humid air out from the area between the living space and the roof to the outside. Remember that your house will generate humidity from showers, baths, cooking and probably the basement. A lot of the humidity will go up in the house and end up somewhere in the attic space. You have to get it out of the house or you will have big problems.
It sounds to me like the prior owner didn't really take a systematic approach to figuring this out. He didn't know what to do so he just kept throwing something at the problem hoping that he would hit the right thing and it would go away. With the cost of energy today, can you afford to do this?
The good news is that the gas and electric companies know that you can't. Almost every one has some kind of program in place to help you solve your problem. Start with them and an "energy audit". If they can't or won't help, find a good pro in this field and work with them. There are a lot of good ones out there and a few that are a little shady, just as in all home improvement areas. Do your homework on the problem and the pro's you are considering.
I know that you don't really want to hear this with your "new to you" house, but somehow or other you are going to have to spend some money. But from the sound of things, I would not be at all surprised to hear that you will get this back in a year or 2 in energy cost savings. The worst thing that you can do is look for another "bandaid".