I have found in the past that xterm UTF support deteriorates badly (compared with say rxvt) with bold fonts (ie can't disable certain random UTF characters properly).

I ended up disabling bold fonts as a result:

~/.Xresources contains some lines including:

*vt100*boldMode: off

 

I think this was with Debian/wheezy, I really should test this out in case it has changed in Debian/Jessie.