3 dimm slot problem...

hi i have a GA-P35C-DS3R ( rev 2.0 bios f12e ) mobo installed with 3 x 1 gb crucial ballistix 1066 ram occupying dimm slots 1, 2, and 4 when i insert a fourth i gb ram into slot 3 the pc crashes i have tried mixing the ram in different configurations with the 3 slot vacant and it boots every time.. each stick of ram has been tested with memtest 86 and all has passed...im not sure if the 3rd dimm slot is faulty or the board has a problem with 4gb ram installed... i had previously had 2 x 1gb and 2 x 512 mb corsair xms 6400 ram installed and i had no problems...has anyone experienced the same problem.....i have a q6600 cpu running a stock speeds , 36 gb raptor , 600 watt ocz psu, bfg 8600gts graphic card running xp pro...thanks in advance...