I re-ran in 2.7.3. First time, B froze at 81, second time neither froze for a few minutes until I used task manager to stop. I then remembered that accessing the gui from a 2nd thread is not supported.

Quoting from msg179101 on #16823, which I turned into a doc issue:
'''
What you are doing appears to be unsupported (invalid). From
http://www.astro.washington.edu/users/rowen/TkinterSummary.html
"all Tkinter access must be from the main thread (or more precisely,
from the thread that calls the mainloop). Violating this is likely to
cause nasty and mysterious symptoms such as freezes and core dumps."
'''
You got the unpredictable random freeze part. But perhaps the quote is wrong (#11077).

But to be sure that was the problem, I reran with B's gui access commented out. It stopped again as about 500. So there is a problem with just printing.

I tested 3.3 and stopped it with TaskManager at 18000 and A still running too.

> Is this issue reproduced with issue9290 patch?

That was just last January. Unless and until I can get new executable with a working tkinter, I cannot test that and other recent patches. Let's hope that 3.2/3.3 still run correctly.