(Trying vehemently to ignore Mr. Trollmaster...)
The "Funny" thing is that there shouldn't be any connection between Heretic II's save games and memory. It just replaces the function pointers with numbers (based on a table) and dumps that to a file. On load, it replaces the numbers by function pointer again (based on the same table), so there should be absolutely no connection to available memory...
Bottom line is I don't have any idea why the game would crash with more memory - you'd normally assume the opposite.
I'm sorry I cannot help you there. It's exceedingly annoying to have the game crash on the last level (I have played the whole game, so I know how hard it was especially in the last few levels). I normally would say it is coincidence, but since the same thing happened before it can't really be unrelated. but like I said I wouldn't know any reason how this could happen.
I'll scan the source code again for clues, but I'm afraid that is all I can do.
Best regards,