OK,
I do the "haze" thingy on my HS
I definately spread the AS5 with a CC or piece of plastic. I use only about 1/2 grain of rice, not 1 and 1/2.
I clean the shiite outta my HS (and cpu) with 91% alcohol.
Look to see how smooth the surface of the HS is. If it's got "ripples" or ridges, it's not gonna make good contact unless you lapp it (sand it smooth with super fine-grain grit sandpaper).
Check your AS5 to make sure the silver and other aspects haven't seperated from sitting too long.
Your mention of the back plate concerns me. You need a tight fit (pressure) between the cpu and HS.
Bottom line is that the thermal compound (AS5) is only intended to fill in the microscopic gaps in the metal of the HS. You want as thin a layer as possible. And any foreign matter, such as the oil on your fingers, or oils in nail polish (acetone) will fill in those gaps and prevent proper heat transfer between the CPU and HS. If your CPU is hot, and the HS is not, then you may not be getting good heat transfer between the two.
A qiuck way ( or I should say cheap way, you still gotta go through the hassle of doing the work) to tell if your AS5 is bad (seperation of components in the compound has occurred) is buy the cheap stuff from an electronics store (Eg, Radio Shack, it will look like a tube of white grease and cost $2 or less) and apply it. Its easier to apply a thin coat of that than AS5. See if there's an improvement. If so, somethings wrong In my experience AS5 gets better over time, the cheap stuff doesn't but still does just as well initially (it just won't ever get "better").
There should be some mention here that if your BIOS are reporting the temps incorrectly, it's gonna shut down from form the heat over-protection. AFAIK, all other temp monitoring proggies just use the same BIOS read-outs to report temps. Sometimes, updated BIOS can take of such temp "mis-reporting".