You mentioned the included CF card, I’ve seen some posts regarding some of the factory CF cards having a problem which causes boot issues or other weirdness. I think this thread mentions several cases of it, maybe check there for a clue. You may have to read down a bit though, someone mentions the graphic image printed on some of the “known problematic” cards so you may be able to compare that against your own.
A checksum error (as a general statement aside from OT) usually has to do with the BIOS being corrupt or some other error in communication between cmos and bios, but in this case if I’m guessing it’s likely some indication of corrupt or incorrectly configured bios.
There’s no question that it shouldn’t be like that, but as to the reason? Can you try a different CF card? Maybe see if you can buy one secondhand or on Amazon with a return policy? Just don’t want you to spend a bunch of money trying things based on a hunch. Anyhow, see if that thread has anything useful, sorry though I probably don’t have any other guesses.
I did read that you previously tried it without the card and still had issues, I’m not ignoring that, just saying that since the problem went away and came back and the card is a common factor for you it might be worth looking into.