The 2.6.9-22 and the 2.6.9-34 kernels have memory bugs. They use ram too conservatively. Red Hat is working on a fix for update 4 of RHEL 4.
The 184.108.40.206 kernel is stable and is what airwave currently recommends.
There are two ways to downgrade. If the 11 kernel is already installed but is not the default kernel you can edit /etc/grub.conf and put the 11 kernel at the top of the list.
grub.conf should look something like:
# grub.conf generated by anaconda
# Note that you do not have to rerun grub after making changes to this file
# NOTICE: You have a /boot partition. This means that
# all kernel and initrd paths are relative to /boot/, eg.
# root (hd0,0)
# kernel /vmlinuz-version ro root=/dev/sda5
# initrd /initrd-version.img
title CentOS-4 i386 (2.6.9-11.ELsmp)
kernel /vmlinuz-2.6.9-11.ELsmp ro root=LABEL=/
title CentOS (2.6.12-1.1381_FC3smp)
kernel /vmlinuz-2.6.12-1.1381_FC3smp ro root=LABEL=/
title CentOS (2.6.9-34.ELsmp)
kernel /vmlinuz-2.6.9-34.ELsmp ro root=LABEL=/
title CentOS (2.6.9-34.EL)
kernel /vmlinuz-2.6.9-34.EL ro root=LABEL=/
If the 2.6.9-11 kernel is not already installed you will need to put a copy of the rpm on the box. Once the RPM is on the box run "rpm -Uvh --force <kernel>". Then check /etc/grub.conf to make sure the 11 kernel is at the top of the list and reboot. When the server comes up it will load the 11 kernel and the memory issues should be resolved.