1.65v's is the limit, but I am having doubts that is the underlying problem. Either the #3 slot is defective, possibly shorted out be an errant standoff, or there is a circuitry issue between the dimm slot & the CPU. The latter would indicate a bent pin the CPU socket. It's a major PITA, but I would recommend another bright light, magnifier reinspection of the socket pins.
Have you tested each RAM stick individually in the primary dimm slot? Does everything work alright with RAM in just slots 2 & 4?