This may be attributed to your MTU setting on your main eth interface (usually eth1 or eth0). The below example applies to an RH-based OS.
When you are logged into shell as root or a privileged user, type in:
ifconfig ethX mtu X
Where ethX is either eth0, eth1, etc.. depending on your configuration and mtu X is a number in seconds. To see what you currently have it set at, you can run the ifconfig command all by itself and look at the MTU value returned. A good number for googlebot to properly crawl your site is around 500. So, as a full example, if your main interface is eth1, you'd run the following command from the prompt:
ifconfig eth1 mtu 500
To have this take effect on reboot/startup, add the following line to your /etc/rc.local file:
/sbin/ifconfig eth1 mtu 500;