Linux Kernel

Linux Drops to 10% or less of working speed with more than 2GB of RAM.
Red Hat Variants (Cent OS)
8 GB RAM
2.8 Pentium D
SATA devices
Intel 965 MB

If I install with 2GB or less of ram the system works great.
If I install 4GB or more the system runs like a 386. Incredibly slow, but still functional.
If I install with 2GB and then upgrade the ram after the initial installation, everything is fine, however, this is not a practical solution.

All 8 GB of RAM are seen.

OpenSUSE, Solaris and WinX64 runs fine??? Don’t Know Why.

This problem also occurs on an ASUS P4 Motherboard with 4GB or RAM.

Suggestions?