DDR Trouble

After upgrading the memory to 1.24 GB from 512MB, my computer would not function properly while running some high-end (24 bit)audio editing applications. It would take 3 or 4 mouse clicks to get any reaction and would leave gaps in some multi-track transfers.
The new memory was 266MHz DDR PC2100 RAM (512MB) just like the one that was in there already, and the machine returned to normal after I removed the new RAM.
I tested the Ram in another machine and found that it seemed to be working fine.
Is there a BIOS setting that I am unaware of that might resolve this seeming memory conflict, or am I just missing something obvious.