Glad to hear "chvt" works! My impression is that C-M-F# is normally keymapped to invoke the very same action as chvt. So using chvt is a great way to circumvent a misbehaving keymap. This strongly suggests that Andrew was correct (see above) that something is wrong with your keymap. As Andrew suggests, please tell the output of "xmodmap -pk|grep VT", or if that's blank, tell the output of "xmodmap -pk" itself.