I don’t really know, I don’t want to make fast conclusions as I don’t really understand your cause, but this seems a glx problem in fact.
I understand that: you run glx/motif single-threaded programs, and those programs crash when on some specific situations (Nvidia but no Nvidia drivers…). Your programs don’t share any context (as it’s not possible) of course.
The problem with Nvidia cards could be possible.
Regarding the official glx spec, only glx 1.3 is OpenGL 1.2 compliancy… so… only Nvidia drivers (at my known) support glx 1.3.
Hope this helps.