Context: I developed in Excel XP using extensive VBA code

Problem: The code appears to cause Excel to sometimes
crash. Not all the time.
In Excel 97, this doesn't happen at all.

Error Message: The following is the error message I get
-- "The instruction at "0x00f00179" referenced
memory at "0xffffffff." The memory could not
be "read". An exception "Unhandled Win 32
Exception" has occurred in EXCEL.EXE.

Debug: When I debug in Assembly language, I get
"Unhandled exception at 0x00f00179 in Excel.EXE:
0xC0000005: Access violation reading location
0xffffffff
Assembly: 00F00177 add byte ptr(eax), bl
00F00179 pop ds
00F0017A lock add byte ptr[eax+30h], ah


Would anyone know what type of VBA code would cause this
as I desperately need to fix this problem?