Hmm... so you're not alone, userx2.
I have seen similar things on other forums, just occasionally and temporarily. I frequent a lot of fora (I have a lot of interests) and certainly do not see this issue often on any of them; I cannot even recall the last time I did. I've always put these things down to temporary server problems, browser glitches, cookie problems or just the internet itself being rubbish. I know that does not help you though if it is happening frequently here.
Like BradtheRad, I have a habit of always doing a quick CTRL-A CTRL-C if I've typed a long reply before hitting send. Still, I have never seen the issue here myself and I often have a thread open for a long time before replying to it as well.
Looking through the various user settings here, I cannot find anything regarding login time-out. I do wonder though if the 'always stay logged in' button is not checked, if the system does use a token/cookie to log a user out (as you think you have seen) after a period of inactivity. E bay does that every 24 hours (and it's a real pain if it does it just as I'm about to place a sniping bid!)
If there was anything going wrong with the server, I would expect a lot more people in here saying so.
Sorry I can't be of more help. I hope you get things sorted out and in the mean time, try the work-arounds and don't let it spoil your enjoyment of the best electronics forum on the web.