I've identified a couple more systems that are affected by this issue:
iucode-tool: system has processor(s) with signature 0x00040651
iucode-tool: system has processor(s) with signature 0x000806e9
That second one was actually having trouble with 4.4.0-116-generic (boots fine with 4.4.0-112-generic). I installed the HWE package on the system and it will not work with either 4.13.0-37-generic or 4.13.0-36-generic, but it will work on 4.13.0-37-generic with intel-microcode 3.20180108.0+really20170707ubuntu16.04.1.
I've identified a couple more systems that are affected by this issue:
iucode-tool: system has processor(s) with signature 0x00040651
iucode-tool: system has processor(s) with signature 0x000806e9
That second one was actually having trouble with 4.4.0-116-generic (boots fine with 4.4.0-112-generic). I installed the HWE package on the system and it will not work with either 4.13.0-37-generic or 4.13.0-36-generic, but it will work on 4.13.0-37-generic with intel-microcode 3.20180108. 0+really2017070 7ubuntu16. 04.1.