If I just push the on/off button at that point it shuts off immediately and starts up fine next time, so may not be a problem.
It's not a "problem" if you can be bothered to reach for the power button. Once it says "system halted" it is safe to power it off, by any method.
Otherwise, I agree, it's an ACPI problem, and this can usually be solved by adding acpi options to your kernel boot line in /boot/grub/menu.lst
Kernel boot options