Insulation, Radiant and Vapor Barriers - Attic insulation dirty & falling

This is my first time posting in this forum, so please excuse my lack of knowledge of technical terms.

The inside of the roof in the attic has pink insulation covered by plastic sheeting stapled to the joists. A lot of the insulation has a black sooty coloring to it. Also some of it has slid down leaving the wood inside the roof visible through the plastic.

The house is 20 yrs. old with a steep pitched roof.

Can someone please explain the reason for this and what I should do about it.

Thanks !!

http://i371.photobucket.com/albums/oo157/StanFromVan/Attic7010.jpg

http://i371.photobucket.com/albums/oo157/StanFromVan/Attic7012.jpg

First of all what are you using your attic for? Storage? Normally the insulation goes on the floor of the attic not the rafters.
About the black stuff: i wanted to say mold but the wood under the plastic looks clean so ill say its dirt that the fiberglass has filtered from the conditioned living space air leaking thru your untaped and not sealed vapor barrier which then goes out your roof. (i even see a hole or 2) Also if you have soffit vents on the edge of the attic air could be getting thru and again the FB is filtering it. Likely both are occurring.
Probably this fiberglass up there isnt insulating a thing at all if the attic is unheated. If you need this space for storage and its heated and want to leave the floor all open you need to redo the fiberglass; leave a space next to the rafters for venting using polystryene rafter vents then the FB and finally put up a proper air barrier like with the pink foamboard nail it to the rafters and seal it with sprayfoam. You likely still wont have enough space to get the proper insulation for your climate if its a cold one. Thats why most people put the insulation on the floor of the attic and build up a floor for storage.

Besides storage, we have our gas-fired heating boiler and hot water heater in the attic. There are some vents in the attic.

There is also a plywood floor with fibreglass insulation underneath. You think I should rip out all the insulation in the rafters??