Problem with 2x512Mb DDR memory on Intel 915GAGL motherboard and Fedora Core 3

Hello, We have recently trialled installing Fedora Core 3 on an Intel 915GAGL motherboard. When the system is operated with a single bank of 512Mb RAM, the system operates very fast, and as expected. Boot time sub 60 seconds. Application operation runs very fast. Installation of a second 512Mb RAM bank, so that the system can run as 1Gb RAM, processing speed drops remarkably. Boot time over 5 minutes, and applications start to run visibly slower. (We have some custom code, consisting of numerical simulation code writen in C, that we have timed on both systems) Have tried disabled Hyperthreading within BIOS, but still system boots slowly with 1Gb RAM (and the application execution times are slower by a factor of x3 to x5) The kernel installed is the 2.6.9-1.667smp kernel that comes default as part of Redhat Fedora Core 3 installation disks. Are there any known issues with Redhat Fedora Core 3 on Intel 915 based chipsets, or are there any known issues with DDR based memory systems? Many thanks for any advice / help or pointers towards assistance in this. Kind regards, Andrew McGhee