StreamWrite/Read 20sec lag on file content

[Philip K]

Using C#, XP and 2 threads in a single App.
Using filestream to open file with shared read.

I have one thread writing data every 2 seconds (open,
StreamWriter.WriteLine, flush, close, sleep 2sec)

Other thread reading every 5 seconds (open, StreamReader.ReadLines while
Length > position, close)

However while the read thread is actually run every 5 seconds, it does not
display the entire file content for at least 20 seconds, as if the file
length is not being up dated after the flush ?

The disk has write cache enabled, but I expect the OS to tell my reader the
state of the file from memory even though the physical disk may not be up to
date.

What am I doing wrong here ?