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Someone may find the new stress.valgrind.stderr interesting, but - since I've never used valgrind - it doesn't mean much to me.
I _expected_ you'd run the little stress program under a debug Python and without valgrind, since that's the only combination you've tried so far that showed a definite problem ("pad leading pad byte" death, or the segfault in the other issue you filed).
But it doesn't much matter - this is all just thrashing at random, yes? You need to find a reproducible test case, and/or try different hardware. The little stress program may or may not provoke an error under a debug-build Python, and may or may not require increasing N (to consume more RAM).
If it does provoke an error, the next thing to try would be to write a little program that just writes 0xfb across a massive number of bytes, and then reads them all to verify they're still 0xfb. Or write one like that now, and preferably in C (it may matter how quickly the bytes are written - and it may not matter). But at this point youj're starting to write your own memory-testing program.
In any case, there's really no evidence of an error in Python so far. Yes, Python has _detected_ a problem in some cases. But without a reproducible test case, I don't see that there's anything more we can do for you on our end - sorry.