Random, PHP out of memory errors on /wp-admin/

I have Apache + XCache + WPMU + buddypress + W3 Total Cache and few plugins.

Generally all is working fine, but sometimes PHP runs out of memory when accessing /wp-admin/ and the culprit line is "always" the same:

Fatal error: Allowed memory size of 67108864 bytes exhausted (tried to allocate 8193 bytes) in /wp-includes/functions.php on line 4455

This is the code from in functions.php around line 4455:

function get_file_data( $file, $default_headers, $context = '' ) {
	// We don't need to write to the file, so just open for reading.
	$fp = fopen( $file, 'r' );

	// Pull only the first 8kiB of the file in.
	$file_data = fread( $fp, 8192 ); // <-- THE LINE !!!

	// PHP will close file handle, but we are good citizens.
	fclose( $fp );

How can php get out of memory when reading only 8 KB of a file?

Anyone has an idea where should I dig?