Output Compression stops working after a while

I enabled output comression in php.ini with zlib.output_compression = On zlib.output_compression_level = -1 also, I loaded the gzip module in httpd.conf with LoadModule gzip_module modules/mod_gzip-1.3.19.1a.so Now, when I restart Apache, I get compressed output. But after an hour or so Apache stops compressing the output and sends it uncompressed. I check compression with phpinfo() and also by plain browsing the difference in speed is obvious. Any ideas what could cause this??? Any input appreciated! -Zavier