Vanilla minecraft. Hard difficulty. No health regen. World border.

Overworld does not crash.
Nether is 95% slabbed inside world border to control pigmen spawns.
Nether did not crash until making zombie pigmen hostile.
Nether now repeatedly crashes once every few minutes.
Crashes accompanied with several server warnings "Can't keep up! Did the system time change or is the server overloaded? Running XXXXms behind, skipping XX tick(s)"

EDIT: Crashes caused by a single server tick taking more than 60 seconds. Server assumes crash and closes out server.
EDIT: Additionally, crashes and server warnings were more frequent when out of reach from pigmen, either due to high elevation or blocking myself in a box.

Go back into overworld, no hostile pigmen, no crashes.

EDIT: Momentarily set world to peaceful while in nether to eliminate hostile pigmen, set back to hard. No more crashes in nether.