Steve: Another possible system affected where disabling microcode loading appears to have fixed it. I'll leave it to you to decide whether that is a duplicate of this issue though:
LP: #1829402 "Purple screen hangup during boot"
Tom and myself have spent some considerable time with the affected user on IRC testing permutations so I'm confident it is the same issue.
Steve: Another possible system affected where disabling microcode loading appears to have fixed it. I'll leave it to you to decide whether that is a duplicate of this issue though:
LP: #1829402 "Purple screen hangup during boot"
Tom and myself have spent some considerable time with the affected user on IRC testing permutations so I'm confident it is the same issue.