I have no idea how you weird ARM people configure your displays/monitors, but from what I read in system info, it looks weird to me having two monitors set in kernel cmdline, while using only one monitor (IIUC)
If this is not some hacky method Pi-xxx uses to make monitors work, you might want to try removing the non-existent monitor from command line.
There are also some possibly relevant/useful commands in the linked wiki post, but I don’t get the command term. Are they actually commands you run in terminal, or config values?
Anyway, since the one present monitor advertises a min resolution of 720x400
, it could be that the lower res in cmdline (640x480@60
) creates the wobbly message for a fantom monitor .