There are several places in pcvt_kbd.c which use the CONTROLLER_CTRL
port and then read from the CONTROLLER_DATA port, and only one of
these had a delay(6).  I added delay(6); before inb(CONTROLLER_DATA)
in each other instance and the problem is now fixed.

This was in the version of pcvt included in netbsd-current.  I hope
both the netbsd and pcvt maintainers will include the fix in future
versions.