I have a Abit SH6 (i815E chipset) mobo, had 256mb installed purchased another 256mb from different place,

Dimm 1 128mb pc133 2 banks
Dimm 2 128mb pc133 2 banks
Dimm 3 128mb pc133 1 bank
Dimm 4 128mb pc133 1 bank

bios sees all the memory as 512mb, but have been having trouble running 3dmark2001, error message about corrupted textures which after checking the FAQ, suggested problem with the memory sticks, ran norton utils diagnostic's which also turned up and error, removed 256mb everthing runs fine including whichever set of 256mb I use just not together any idea's or advice to find a way so that I could use all of it without error?