I ran the 2.7 file (on 2.7) and after several minutes, threadA stopped printing, but no error message.

Antoine, could improvement from 2.7 to 3.2 have anything to new with the new implementation of the Global Interpreter Lock?

Scott, if you can do your project with 3.2, I strongly recommend it. Even with 2.7, one can run files outside of IDLE even though you edit with IDLE. You just give up the F5 convenience. Or, I believe there are other editors without a shell which do so directly. Or, you could write a dp() (debug print) function that writes to a scrolled window in your toplevel app window (where running numbers appear now.