Newbie: Terminal window erratic behavior with Nethack

[stan mcintosh]

I'm having trouble getting consistent terminal window characters with 
Nethack; sometimes it plays fine, sometimes the characters seem to be 
shifted into the upper half of the extended ASCII character set.  After 
quitting, the character map in the terminal window is still screwed up, 
with directory and typed information being from (seemingly) characters 
8F and up.  Any ideas as to what I have misconfigured?

Thanks.
stan