GPIO madness.
Dear Lazyweb, riddle me this:
1. Boot Pi with HDMI monitor attached: works fine.
2. Boot without monitor: all GPIO inputs "flap" about once a second.
3. Plug in monitor: 10 seconds later, inputs start behaving normally.
4. Unplug monitor: remains good. Until reboot.
UPDATE: FFS people, read the replies before responding. Do not be the *seventh* person telling me that HDID dongles are a thing that exists. I know! I know!
@falken It is a text mode machine with no GUI installed.