Bluetooth keybaord isnt found on iMac since high sierra upgrade

Since upgrading my iMac to high Sierra when booting up it fails to pick up the keyboard. I have to remove the batteries and put them back in again, which seems to put it in a longer discovery mode - evebtually then it gets picked up.

It wouldn't be so much of any issue but a bigger issue is that the iMac also now keeps randomly powering down and I have to hard reset about 4 times again. Something it also did with El Capitan, but not so much with sierra.

Any ideas??