To answer your original question, no I don't think the P5WD2 has an inherent thermal prob regarding CPU temp. My 830 ran 36c at idle with my WC which would prolly traslate to about 48c if it were on air. The typical causes af high temps like that are ppl that have not seated the haf correctly. Some ppl, after they pushed the pins in, turned them thinking that would secure the hsf better. It infact releases it. Also som ppl have turned the pin to the wrong side before pusing them in and that also resulted in a bad contact with the CPU.
Another issue could be your psu. It only has 28a on the 12v line. Depending on how much the other devices pull, it could be running short of juice or it could be going south.
You have a good number of case fans, and you sure seem like you are knowledgable in that area, but have you tried it with the side of the case off. If it is a circulation issue you will see the temp go down right away.
I know you are confident you attached the hsf correctly. But more times then not that is were the problem is.