RAM Problems

I’m running Windows ME,with an Asus MB,Pentium 111 933 chip,Decided to upgrade RAM,from128mbs, to 512,But PC would not recognise other RAM chips.Removed all,installed one at a time, ran Norton Utilities to check each RAM,in slot one, all proved OK,reinstalled all, still would not check out.BIOS aknowledged 512 mgs, but PC was VERY slow, and when Norton Utilities was initiated, RAM would not pass test.Swapped two 128 chips for one 256 chip, installed, worked/tested ok, installed another former 128 chip, would not test ok, PC slowed down.
What can I do, to get 512 mbs installed and running correctly ??????