Save-game/reload-game bug in MEPHISTO.WAD (Master Levels):

The "Boss" entity in Doom II-based games (Doom II, Final
Doom TNT/Plutonia) can create a save/load problem for that
level in the game.  If you save your progress on that level
after the Boss has detected (seen/heard) you, that saved
file may cause a crash if you start a new game and reload
that file.

The best way to prevent this problem is to save and load on
that level only during a single unbroken gaming session.
Or, save your game on that level only at a point before the
Boss detects you.

If your save-game file crashes when you load it, then I'd
suggest starting a new game at the same Skill level, and
use the "idclev" cheat code to jump ahead to that map; then
immediately save the game, overwriting the corrupted file.
Details on the idclev code can be found in the DoomCode
document.

Good luck!