systems hang on intel microcode update

The intel-microcode was updated last night on Kubuntu 17.10 from:
    3.20180108.0+really20170707ubuntu17.10.1
to
    3.20180312.0~ubuntu17.10.1

This resulted in *both* of my systems hanging shortly after boot (random times, from almost immediately to ~1 minute). (MS Windows still booted OK on both systems).
Eventually I managed to get both to start in single user mode and downgraded to the previous version (download from the website onto a USB stick, mount it and install with dpkg). This has fixed the problem.
The two system are running these CPUs:
    Intel(R) Core(TM) i5-2500K CPU @ 3.30GHz
    Intel(R) Core(TM) i5-4330M CPU @ 2.80GHz

I have another system running 18.04beta. It seems OK (and only has the 3.20180312.0 available as an option!). It is:
    Intel(R) Core(TM) i5-5250U CPU @ 1.60GHz

(I first downgraded the plymouth update, but that didn't fix things and I'm now back on the latest version of that, which is working OK).

ProblemType: Bug
DistroRelease: Ubuntu 17.10
Package: intel-microcode 3.20180108.0+really20170707ubuntu17.10.1
ProcVersionSignature: Ubuntu 4.13.0-37.42-generic 4.13.13
Uname: Linux 4.13.0-37-generic x86_64
ApportVersion: 2.20.7-0ubuntu3.7
Architecture: amd64
CurrentDesktop: KDE
Date: Fri Mar 30 13:32:05 2018
InstallationDate: Installed on 2017-11-19 (131 days ago)
InstallationMedia: Kubuntu 17.10 "Artful Aardvark" - Release amd64 (20171017.1)
SourcePackage: intel-microcode
UpgradeStatus: No upgrade log present (probably fresh install)