[CentOS] Centos 6: Dell GX270 fails to power down (but reboots)

Hi,

am using latest C6 on a Dell GX270 with latest Bios.
It fails to power down but restarts when running
$ sudo poweroff

I tried some google hints for kernel boot option:
acpi=force
no help, looks like acpi is working anyway when looking in dmesg
acpi=off
makes e1000 nic fail to initialize

Any suggestions?
Rainer