Tried a lot of options described here, here, here and on other forums but neither worked for me .
Tried a number of grub parameters including acpi=force reboot=acpi
as well as all sorts of reboot
params as described in this answer.
There are no problems with reboot however, only with power off (system shuts down to a black screen and then I have to hold the HW shutdown button to switch the computer off)
Any suggestions/thoughts on how to fix this specific problem?