sudden disappearance of text...
 

while typing text in yahoo mail-compose, the text sometimes automatically gets disappeared.
Why does this occur?

To recieve any useful advice, you'll need to give much more information - what version of which browser are you using? Does this behaviour occur when using any other web sites?

it's not actually happening in my computer. It happens in a friends PC. And i've heard the same case from a few others as well(Windows Users, with 'internet Explorer'), it seem to happen while composing mails in 'free mail accounts' (yahoo etc.) .

I believe you may find there are many forums in which you'll find many more experienced Internet Explorer users than on linuxquestions.org.