I have the same board, its the capacitors. This board uses solid capacitors, and low end solid capacitors are rated to 105C (which is most likely what is used on this board). If you are reaching 91C at idle, then that is quite high. Mine reaches 75C when running prime95. The capacitors have a large heatsink on them and are next to the rear i/o. Try changing your fan configuration so the rear fan blows inward, onto the capacitors heatsink. If you are at 91C at full load, its not much of a problem, but it will raise the ambient temp in the case which can cause other temps to rise.