Memory CAS problem

Hi!
I have a fujitsu-siemens amilo 1424, with 2x256MB pc2700 ram.
I cannot set CAS by bios, since it's automatically detected, and from
memtest I get CAS: 2.5/3/3/10.
I got from ebay 2x512MB PC2700 CL2.5 ram, but when I use it, memtest
indicates CAS 2/3/3/10, and the test fails.
I performed the test mixing old and new banks (therefore with
512+256MB) and everything is fine (CAS 2.5/3/3/10).
Why is my motherboard detecting the wrong CL? is my new memory faulty?
is there a way to force it to 2.5 anyway, even if my bios doesn't
allow me to change it?
I have seen the software wpcrset, but I am looking for a more
definitive solution.
Thank you for your help,
Paolo