Worse memory timings with better memory sticks

Hi,

I have a GA-8KNXP (rev 2,BIOS FB)with P4 2.8C(800MHz). I had two
sticks of Kingston KVR400x64C3/256 (3-4-4) running at CAS 2.5 without
any problem until today (DIMM overvoltage +0.1V).
I bought recently a pair of Kingston KVR400x64C3A/512 (3-3-3) and
replaced the older ones. Unfortunately I couldn't run the newer ones
at CAS 2.5. I get "memory could not be read" error in games when I try
it.They run fine at CAS 3.
All the other settings are the same. The first pair had Kingston
modules, the second ones have not.

Could the new memory sticks be faulty?

Thanks for any help.

Orhan