output redirection delay

I’m using pgf90 on an opteron.
Whenever I’m redirecting the output to a file, there is a delay.

Here is what I’m trying to do:
./fortran_code > output.txt &
The code is running fine, but the output.txt file is being updated with a delay (about 15 minutes).

If I’m executing the code simply as
then there is no delay and all the output data are shown immediately on the screen.

I tried sync command, it doesn’t help.
How can this be fixed? I need immediate output to the file, wihout using a buffer.

Thanks -