On one of my systems, I can't select an item to boot

[bbertrand007]

Display is pale are garbled - but readable.  Problem is that I can't use the arrow keys.  If at the menu, which is a bit garbled, if I try to use the down arrow, it is like the menu repaints itself; the menu is not scrolled and it is impossible for me to "scroll" to select the target ISO.

This only occurs on one system, booted in MBR mode.  On my other system it boots fine in either UEFI or BIOS mode.  Pressing the TEXT mode F-key doesn't help.