I've had that happen a few times, and like you said, it's usually when I'm posting a long reply or new post. When I'm doing one that I don't want to re-type I will copy it to a Word file as a back up just in case. I've not found a definitive cause for it but suspect it may be an ISP issue where slow activity causes a glitch in the conection. It sure can be frustrating when you spend 10 or 15 minutes typing a post only to have it disappear.