(VFP 7 SP1 on Windows 2000 or Windows XP pro)

I have to perform 3 operation on a text file.

1 - read a line from a text file.
2 - perform some operations on the string.
3 - write the line in another file.

The text file was really big (950 mb).

The program run fast for the first ~50 mb but after that it start to slow down (crawl).
The computer have 1 gb of memory an 100 gb of available disk space.
From the task manager I can see that only around 200 mb was used during the operation.

What is the best way to open the files? buffered/unbuffered?
Flush when I reach a certain point in the file? every 10 mb?
How can I optimize the process to use all available memory and speed up the process? some sys() function to use?

TIA :-)