Slot 1 boots ok, slot 2 boots ok, slot 3 no boot, slot four no boot, slots 1&2 boots ok. With slots 1&2 filled, Putting an DIMM in the 3rd slot no boot, 4th slot no boot, 3&4 no boot. I've done this with each Dimm and get the same results.
What is the manufactures recomended voltage? have you tryed to up it?
You can't change the voltages on the board
Is that the Asus A8V-VM ? If so In the BIOS with the AI overclocker tune set to Manual you should be able to change the voltage under advanced / jumper free config / ddr ref voltage it is on pages 2-17 and 2-18 of your manual.