Actually now I found the real cause of all the problems. It's the headers generated by send_file in filelib.php. I simply commented the generation of the headers for html pages (not for raw binary data), and everything works now. It seems, that after upgrading Windows 2003 to SP2 some of these headers cause the restart of page load, if it is loaded for too long (for speedy connections that does not happen).
Moodle networking (MNet)
Big pages cause the load restart for slow connections
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