Deadly "Screen of Twos"

[mtm4@rsvl.unisys.com (Mike McCormick)]

I am running a ProDOS8 application (written in Applesoft plus assembly patches)
that has worked wonderfully for years on my IIGS.

Suddenly it has developed a problem when I load a certain large file (it's a 
full screen editor program).

The symptom is the screen is full of "2"s, the speaker is clicking, and the 
keyboard is hung.

Control-Reset breaks the hang, and momentarily restores the text screen into 
view before the machine reboots.

I suspect a clobber of some crucial bit of ProDos8 memory?...

Can anyone guess what could cause this odd behavior?

  _________________________________________  
 / Mike McCormick                          \ 
 | business: mtm4@rsvl.unisys.com          | 
 | home:     m.mccormick2@genie.geis.com   |
 |           70714.551@compuserve.com      | 
 |~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~|
 | "Follow your bliss, and doors will open | 
 \  where there were no doors before."     / 
  -----------------------------------------