Problem writing program using large data files under Windows XP

[stroy]

I have been trying for some time to write a program to process databases contained in structured ASCII text files.  The program (1) opens a standard Windows dialog box for the user to select a input file; (2) reads the input ASCII text file and generates an in-memory index; (3) uses the index to process the information various ways, with a dialog box showing the progress of the processing; (4) opens a standard Windows dialog box for the user to select a file (new or existing) into which to write ASCII file reports based on the processed index.  The program behaves exactly the same regardless of whether written in XDS Modula-2 or XDS Oberon, both under Windows XP.  I.e. it works fine, exactly as described above, for small data files, e.g. about 30 or 40 kByte.  For a large file, e.g. about 3 MByte, the program displays the input dialog box OK, the display of the in-process dialog box is erratic, and the call to the standard Windows dialog box to select an output file fails, with an error message instead.  Only the size of the input file, and the corresponding size of the in-memory index, is different.  I went from Modula-2 to Oberon to take advantage of memory garbage collection, but this does not seem to make ay difference.  I would appreciate any suggestion or advice on what to try next.