file upload

I'm using xmlhttprequest to upload images to my site using POST method and PHP script to recieve it.
For the most part it is working fine, however, trying to upload any image larger than 300k usually fails.
That is to say that sometimes it does work but most times it just keeps trying indefinitely with no response from
the server.

htaccess includes:

# av:PHP-upload
php_value upload_max_filesize 20M
php_value post_max_size 20M
php_value max_input_time 300
# PHP-upload

What might be the problem?

Thanx in advance

[Gianluca]

Looks like there is no issue on server part, have you also tried with another connection?

Thank you for your reply. I will test on other connections.