null character without reason in written files

[unomas]

Hello,

Analizing the files produced by program I compiled with gfortran I 
discovered that sometimes data was corrupted with null characters. It 
happened in several files, but not always. (The program uses a random 
number generator, so I cannot reproduce any particular output.) Produced 
files are big, between 20 and 300 MB.

Produced files are like this (all is fine except null characters, \@):

    1.3333333358168602       -1.0000000000000000
    2.6280343979597092       -1.0000000000000000
    876.20208282163367       -1.0000000000000000
                     [...]
    4.0368796885013580       -1.0000000000000000
    352.90456\@\@\@\@\@\@\@\@\@\@\@\@\@\@[...]\@\@    3.4207813031971455 
      -1.0000000000000000
    1.1111111119389534       -1.0000000000000000
    2.6666666790843010       -1.0000000000000000
    1.1111111119389534       -1.0000000000000000


[...] stands for more similar to what is backward.

What do you think that can be the reason?

The compiler is old :
GNU Fortran (SUSE Linux) 4.5.1 20101208 [gcc-4_5-branch revision 
167585]