It's done 🙂.
From time to time, the sticky piston block generates a force above him instead of glass that allows the elevator.
When the bug appears, the sticky piston no longer works due to the replacement of the glass block by a block of strength despite the fact that the redstone torch below it continues to feed.
Do you need the crash report when the bug is present (it was not present at the moment as I corrected through administrator and destroying the block strength)? If yes, I will reappear as soon as the report.
Confirmed for Vanilla 1.7.9