php script is not executed

Hello:
I have installed phpmyadmin on a Debian server with apache, and modified the code quite a bit to suite my needs, and I have found a strange problem:

When I login and perform a table search for which there are no results, after about 3 seconds of trying to load the page, firefox offers to open the tbl_select.php instead of showing the interpreted page.

However, if I first perform a search for which there *is* a result, everything works as expected, and after that any search works fine.

I thought it could be a timeout problem, or a script size issue, but the delay is of only 3 seconds, and I have touched php.ini to increase these parameters:

;;;;;;;;;;;;;;;;;;;
; Resource Limits ;
;;;;;;;;;;;;;;;;;;;

max_execution_time = 100 ; Maximum execution time of each script, in secondsmax_input_time = 60 ; Maximum amount of time each script may spend parsing request data
memory_limit = 32M ; Maximum amount of memory a script may consume

Timeout in Apache's httpd.conf also seems big enough (300seconds)

Anyone has any idea of what could be causing this problem?