As I suspected. Application bug, not the computer's fault or the OS's fault. Which application are you using to type your emails and BBS posts? Web browser, right? Which one?

First try making sure that it's not a problem with having too many tabs open at once. Close all tabs except the one where you're typing the message. Does the problem go away?

Next, look to see if the web browser has any add-ins which may be interfering. Try disabling all add-ins and plug-ins and extensions and whatnot and see if the problem goes away. If so, then re-enable the plug-ins one at a time until the problem recurs. Then you've found the culprit.

If that doesn't work, search the web to see if other users of that web browser have encountered the same problem. Perhaps it's some kind of configuration setting, or known issue.

Finally, try a different web browser to see if the problem is solved by a different web browser. I list this one last because I know that for some people, their choice of web browser is non-negotiable.