SDRAM: pair of modules don't like each other, prefer to work alone

[Spright]

I'm experiencing the familiar problem in which a pair of modules report errors in Memtest86, but each one alone doesn't.

2 x 512 MB SDRAM DIMM 168 pin, PC133 MHz; Pentium 4, 1.8 GHz. I believe the two modules are identical, but don't know how to check.

I played around with BIOS, and was able to SOLVE the problem by setting "Memory frequency" to PC100 instead of PC133.

I'm quite thankful that it works now, but have two questions:

1. Why wouldn't it work at 133, although the modules are 133? Is there a way to address this issue? Any other BIOS setting that may help? It seems, by the way, that I can't change the voltage in this BIOS.

2. I have another module of 256 MB, PC133. It works fine with one of the 512 MB ones, resulting in 768 MB of RAM. Now, which setting will result in a faster computer, 768 MB at PC133 or 1024 MB at PC100?

 

[Vic 40]

You can look on the side of the modules to check whether the modules are the same or look with cpu-z which is freeware.
Get the more ram at a little lower speed.
How to get those to work at a higher speed might involve changing settings like voltage,which you say you can't or maybe set other settings like latency's as rated.Don't know what you do with that oldtimer,but just leave it as it is would be my advice since working fine now.

 

[Tradesman1]

What mobo, CPU and the model #s of the DRAM? Do you have the latest BIOS?